One year old tree (known as a maiden)
This is ‘easy peasy’ as you simply cut off half of the branch.
These generally have three or four branches. Cut strong, long branches back by half and any short or thin ones by two thirds. The shorter, thinner ones need more drastic pruning to encourage strong new growth.
Three-year-old (+) tree
Cut off a third of the current year’s growth from the main framework branches. You don’t need to do anything to the side branches growing off the main branches unless they’re crowding each other out. In this case, remove a few to allow good air flow around the remaining ones.
With all types of pruning, ensure you use sharp cutting tools to not only make your job easier but to avoid ripping the stem which could allow diseases to enter.