On a trainer it would be better to have the leading edge at the tips lower . The lower leading edge at the tip will reduce the tip stall tendency ,this is usualy refered to as washout. Raising the leading edge will increase the angle of attack and cause the planes airfoil to stall easier.
It really doesnt matter if you use heat on the top or bottom first but both must be done to hold the change effectively.
Eyeballing the warp on a trainer wing is not difficult to do but using an incidence meter will get you where you need to go with less guess work . If you are going to continue to assemble or build planes an incidence meter is worth adding to your tool collection