I am not sure which covering you are using, but I use monokote.

1) Unroll your monokote, and drape it over the turtle deck in the lengthwise direction of the roll (Monokote shrinks more in the lengthwise direction compared to the width of the roll, and by doing this, there will be less sagging in the open bay areas when you shrink it down. Same thing applies to covering wings with open bays)
2) Use a felt pen to mark the corners of a piece that is approx 2 inches larger than the deck
3) Cut that piece out
4) Remove the backing (you'll find it very easy if you use the sharp tip of your #11 x-acto and "dig" a little into the backing sheet) and place the monokote over the deck in the proper orientation, leaving 2" margin all around
5) Use the half point rule, and tack down the canopy end first, then stretch the monokote to the tail end and tack down. Stretch to the sides and tack down, always using the half-point rule to minimise sagging and wrinkles after shrinking
6) Iron down all the edges
7) Trim the film leaving 1/8" to 1/4" margin all around
8) Seal down all the edges and use your sealing iron or heat gun to slowly shrink the covering drum tight. Be careful with the heat gun near the edges as the seams will lift if overheated ! You may want to cover the seams with a damp cloth before using the heat gun

That's all to it. If the deck is sheeted, it will be slightly easier. But why add weight to your plane ? Good luck.........