I've done this with my Apprentice, Timber, and Beaver but for some reason works better on the Valiant 1.3 with SAFE Select/Self Level On:
Align Valiant on a short grass runway directly into wind. Set T/O or Land Flaps. Slowly advance throttle to where Valiant's thrust starts to overcome the drag of the wheels in the grass. Call this "minimum break away thrust". For the Spektrum DX 6,7,8,9,18 transmitters note the numerical value on the top line of the on screen display. This will vary according to grass height and other factors. Today I used 67 % for "min break away thrust". My student's DX9 used 65 %. When the model starts moving only move the rudder stick left or right to maintain runway heading. Try not to add power as the added P effect and left prop thrust will require more rudder correction. Your Valiant will gradually start rolling and slowly accelerate and rise off ground (R.O.G.) in about 50 feet. The slow motion climbout is impressive and fun to watch. With practice I was able to make straight takeoffs without using any left or right stick. Of course you can abort or add power or intervene with a wireless buddy box. Using low power extends flight time and adds valuable training experience. With E Flight 3S 3000 lipos we were landing after 30 minutes with 20% capacity remaining. Full power was used only for 10 low approach and go arounds and occasional aerobatics with SAFE off.