Catching thermals is about carefully watching the model for disturbances caused by the air flowing around it and reacting to them. On calm days the disturbances will be weak and can be discounted as nothing unless you're on top of your game. On blustery days the signs can be violent to the point of trying to crash the model and quick and decisive control is needed both for survival of the model and to maneuver the model to actually trap and ride the lift as it goes by. The one common point is that a trimmed model does not suddenly go out of trim for no reason. The flight path is altered by some disturbance in the air. Often those disturbances are thermals.

First off, if you only fly in dead calm days then it's hard to catch lift. It's there but so weak that a newbie likely won't see it or it may be so weak that it doesn't hold up the model. Look for days where the wind is light to moderate and seems to be variable in both direction and strength. If it's mixed up like that then there's very likely thermals around.

Launch the model and fly directly upwind so the model is about 100 yards upwind of the release point. Now gently turn the model so it flies cross wind but with its nose still crabbed into the wind so that the ground track stays out at the proper distance. Depending on the wind this means the turn may only be a few degrees or it may well be 80 degrees. When the model has moved about 150 to 200 yards off to the side gently turn it back into the wind and keep turning to re-establish the same crab angle for the return. Let it fly past the launch point and off to the other side. When it gets to the same distance on the other side gently turn back and repeat. This is your basic search pattern. Exciting, eh? (I'm Canadian, I'm allowed to "eh" )

The key is to keep it upwind the same distance for now. Alter the direction to keep this. And for all this turning the sticks should only be used with very slight motion. The more you deflect the controls the more drag you create. The key to efficient flying to conserve altitude is to barely move the controls and then wait the 2 or 4 seconds for the model to react. If you're not in lift don't be in a hurry to do anything. Rapid turns and corrections are high in drag and that wastes your fuel (altitude). Also keeping your inputs small means that the air moving the model will be more apparent. And it's the air moving the model around that is your indication of what is happening out there.

The most likely thermal interaction when crabbing back and forth like this is that the windward wing will suddenly lift up and the model will try to turn off the wind. Fight this with a strong turn back into the pushed up wing and turn into the wind. As the model banks over feed in elevator to maintain an even flight speed. Be quick and decisive here. You're potentially in lift and the gentle rule from above does not apply in this. You need to KEEP THE FLYING SPEED NEAR TO CONSTANT. A stall at this point is like loosing the fish off the hook. As you turn into the disturbance watch the model closely. The initial tendency is for a slight lift up due to all the corrections adding speed. But if it was not a thermal these actions will fade away within a second or two. If it is lift you'll see the model start to gain height as it turns into the lift. Let the turn open up and fly into the wind for a moment to see what is happening. If it keeps lifting then fly until you see it start to sag off and then carry through with your turn in the original direction to bring it back around into your thermal circle.

Another way to enter a thermal is nose first. The telltale signs for this option can be confusing as it depends on the model. I've had some models that tend to lift the nose and want to slow down and need a quick push of elevator to punch into the thermal and start rising. I've had others that like to lift up the tail slightly at the same time that they slow down and start rising. Again, the way to success is to fly your model a lot and watch it carefully to see what it's doing.

This is where you're elevator control comes into play. You need to fly a moderately tight turn with a 20 to 30 degree bank angle and use the elevator to restore and maintain a close watch on your proper flying speed. This is hard due to the apparent variations as the model turn into and off the wind. But if you use the cross wind portions of the circle as a guide you should be able to get it set pretty quick. I cannot stress how important it is to master the elevator control when soaring. It is your throttle. But unlike a power model THIS throttle only works if it has airspeed to work with. It is imperative that you use a quick and finely tuned stab of down if needed to kill any tendency to stall or suddenly slow down. If some gust or lift suddenly lifts the nose you need to be quick on the stick to jab in some down to lift the tail and keep the model on an even keel and at a proper flying speed. Thermals are often turbulent and to some extent you need to fight that turbulence to keep the model flying through it. But just as importantly you need to be ready with some up elevator at the first sign of the model wanting to pick up the tail and try diving. A thermal or gust induced dive is like having a gas line on your car suddenly start leaking when you're in the middle of nowhere. You need to "plug that leak" quickly.

But what if the disturbance was not a thermal? Well, you reacted and turned into the wind but as you let the model fly a few yards upwind you see that it was just a little turbulent rotor or gust that caught your wing. So cut that cross wind leg short and carry through with a reversal back the other way. The model will still have a little bit of the upwind turn in it so it's more efficient to just work with the flow rather than fight it. You already lost a precious 10 feet or so dealing with the possibility that it was a thermal so cut your loses rather than loose more by returning to the original track. Damage control is just as important as finding a thermal. Same if you do find lift and start turning only to find that you turned the wrong way or are not successful in locating it within a two turns. Once you realize it's a lost cause just maintain a smooth turn until you come around into the wind. Fly upwind with a slight crab back to the centerline and when back out at the range increase the crab angle to maintain your 100 yard upwind line.
When in the thermal turn watch the model closely for signs that the lift is stronger on one side than the other. Also if you're not centered you'll find that on one side of your turn the lift tries to push the model away by increasing the bank. Use that disturbance to let the turn tighten and as it comes around and is pointed more or less in the direction the disturbance hit open the turn up to fly into the area of sky the disturbance came from. By watching and reacting to this stuff you should be able to center your turn in the lift. This is where the skill and handling comes into play. You need to learn when to let the turn open up and then close it down in order to place the model where it needs to be. But at the same time you don't want to use any more control input than is barely required. This means you need to start letting the turn open up a good 90 to 120 degrees before where you want it and to start closing it up ahead of time as well.

Thermals also come in lots of sizes and flavours. There's the big soft ones where the turn can and must be large and open. There's the little stove-pipers where you either need to dance on your wingtip or fly through it for only part of the turn. And then there's some that try to push your model into a spiral dive and once you're cored you need to be constantly holding some outward control to maintain your position. Then there's those that need to have the model force its way constantly into them. You'll find all sorts so be ready to be reactive to the needs of the moment and always be watching the model for the signs of what the air is doing to it.
NEVER turn downwind unless you think there's a thermal or it's time to come back and land. Always make your reversals into the wind like a sailboat tacking. Gliding is by far the easiest method of learning to fly a model airplane. The models are gentle in the extreme to learn on. However, learning to SOAR is a whole other issue. Gliding is coming downhill while soaring is gliding uphill. Learning to read the air and truly soar is a skill that mixes a wide number of skills that must all come together at the same time and in the proper proportions. But when it all clicks and you find that you had the longest flight of the day in your group and in bad conditions you'll know that you got all you could out of the air that day and it'll be a great day to be alive.