Hi,
I don't know of any documented tutorials here yet, so I will tell you how I go about it

Knife Edge Elevator Coupling

Knife edge is really a pretty easy flight attitude once you get the coupling issues corrected. The key is to get the airplanes really trimmed out for straight and level flight first and then work on knife edge.
Knife edge coupling issues are not only effecting the airplane when it is flying in knife edge, but any time the airplane is flying including when it is in a hover. Getting the plane trimmed out and then mixed out properly will make hovering easier.

1. Once you get the airplane trimmed out and you are ready to fly on knife edge, get up 3 mistakes high and get the plane flying level and parallel to the runway or even better straight into the wind. BTW, its best to trim for KE on a day when there is little or NO wind.

Lets start out flying left to right

2. At about 1/2 to 3/4 throttle, roll the plane on knife edge so that the canopy is facing you and immediately start to ease in just a little rudder.
*If you are rolling to knife edge from upright flight you will always input OPPOSITE rudder to the direction of aileron roll.
* If you are rolling to knife edge from inverted you will always input the SAME rudder as the direction of roll.
3. Do not correct with the elevator on the 1st try, just watch the airplane and see what it does and take a mental note for now.
4. Go back around and do it again, again just watch and makes sure it does the same thing again.
5. Now, go back around and have someone standing there so they can write down what you observe (unless you are sure you can remember) ie Say right knife edge, canopy facing me, the plane is pulling to the belly a little so that they can write it down for you.
Once you are ready, roll on knife edge and as the plane couples, begin correcting with the elevator to hold the plane in a straight line. Make a mental note of HOW MUCH elevator you correcting with and whether it was up or down elevator.

Now, lets fly right to left.

6. Repeat steps 2-5 but flying right to left and record your corrections, if there were any made with the elevator.

Mixing

Lets assume that on left knife edge it pulls to the wheels(belly) and on right knife edge it pulls to the canopy.

7. Now, go into the transmitters mixing menu and look for a Rudder to Elevator mix. If your radio only has the channel numbers then find a mix that uses the rudder channel (4 on Futaba) to the Elevator Channel (2 on Futaba) You might see a mix called !4 to 2. If there is no mix there, you will have to go into the programmable mix (pmix) menu and assign one of them to mix Rudder to elevator by selecting the rudder as the master channel and the elevator as the slave channel.
Master means just what it says, its in control of the slave channel. The Slave channel reacts to what the Master channel tells it to do according to how you set the mix.
7A. Once you find the mix, with your airplane on the ground (and the frequency pin in hand I hope) turn the airplane on and go to your mix.
7B. You should find a place where you can assign a +/- number from 0-100 for left rudder input and right rudder input. One will be for left rudder correction and one will be for right rudder correction.
*You will probably have to actually move the rudder stick to get it to toggle from the left rudder number to the right rudder number so pay attention to which is which. For instance, if flying left to right, when you input left rudder the plane happened to pull to the belly you want to add UP elevator.
* Move the rudder to the left and see which number the mix switches to. That is the one you will want to change to mix in elevator for left rudder .
8. Now that you have located the mix and know which one is for left rudder and which for right you can start programming.
8A. Move the rudder to the left and get on the correct adjustment number for left rudder mixing. Move that number to -5 to start.
8B. Move the rudder and watch the elevators direction of travel. Since on left rudder we said we needed down elevator the elevator should move down. If it went the other way, change the number to 5 or +5. Now, move the rudder again.
OK, so now you have it going the right direction.
9. Remember when you were making a mental note of how much elevator you needed to hold the plane in a straight line? Now we will correct according to that number and it is not an exact science. If you had to make a lot of correction ease the number up to about +/-8 to start. That is a lot is a lot of elevator correction, but it is possible that you could need more. Move the rudder and make sure the elevator is going the right way. If you only needed a small correction start on +/-3 and go from there.

10. Now starting with step 7 repeat the steps for right rudder correction (plane flying right to left)

11. Once you have the mix in and you are sure that the elevator is going the right way for whatever correction you need.
12. Fly the airplane and see if you can now hold a straight line without elevator correction.
12a. If it is still pulling in the same way it was before, add more mix by making the number more + o -, whatever is applicable. .
12b. If it is now pulling in the opposite direction, reduce the mix.
Continue to fly and adjust until the plane flies straight on knife edge.

You can also do this mix on a curve mix so that you can incrementally adjust the mix as you increase rudder input, but for now we will keep it simple.

Knife Edge Roll Coupling

Once you get the elevator mix done, you can now start on the roll coupling mix. That means that when the plane is on knife edge you are making subtle aileron corrections to keep the wings perpendicular to the ground.

1. Fly the plane left to right and do not make aileron corrections once it is on knife edge.
Try to fly all the way from one end of the field to the other and just watch and see what it does. If it continues to roll toward inverted you will need to add left aileron, if it wants to roll back upright you will need to add right aileron mix.
2. Once you have done this test from L to R and R to L, go back to the mix menu and look for or create a Rudder to aileron mix.
3. Just as before, with the airplane on and using the rudder stick setup the mix numbers so that the ailerons move the right direction for the mix you need. Same thing with +/- numbers. If it goes the wrong way go to a positive number. Again, start out small with the numbers I would not go more than 10 to start and adjust from there.
4. Fly the airplane and depending on what it does increase or decrease the mix until the plane flies straight down the field, without wanting to roll over.
Thats it!

Just be sure to remember to make SMALL moves, tiny changes can make a BIG difference, even though it does not look like the surface is moving much. A 1 degree change can make a big difference, all the difference in the way the plane flies.

Guys, feel free to correct me or add to this if you don't like.