okay, here are a couple. first, just stick your fingers straight up and try it that way. 2nd, get a plumb bob and hang it from a decent height, hang it from an eyehook on a beam somewhere. next, tie a string around the very tip (i mean the very tip) of your rudder and slip the string through the eyehook and hang it down. Tie the other end as far forward as you can like the tip of your spinner or close to it. make sure the plane is level, then lower the plumb bob down to where it is just above the fuselage. close all doors and windows, let it sit for a while so that it ends up standing completely still. after a couple minutes, walk back in and mark where the plumb bob is pointing to. this is your cg. see, the cg will want to be directly under the centerpoint becasue of gravity. you can test this by adding weight to the nose. you will notice the plane will slide back a little. this is a great tool also because it makes it easy to laterally balance your aircraft. if it wants to tip to one side, add a little weight to the other side until it does not tip over by itself. this is not really that important for a trainer but when you get into sport planes or 3d planes it becomes necessary.