For your horse to understand that you want to move from trot to canter, it is important that the rider gives the question or aid, quietly and the same every time. Never forget that the horse learns by repetition and memory.
From trot, sit in a good upright posture in the centre of the saddle. Feel that your horse is carrying you along with ” the engine” running energetically. Keeping a steady hand over the wither, slide the outside leg 3 inches back without using it and press the button on the girth with the inside leg.
A young or uneducated horse is likely to try and run into canter and if this happens, the rider needs to close the base, bringing the hindlegs closer to the front legs for a step or two and then apply the aid.
Repeat the transition several times on both reins.