The rind graft is a useful technique for grafting over a sound but unwanted apple tree to one of a preferred variety. Same for pears. You may wish to do this after moving into a new home where there is an apple you don't like, or if you have been mis-sold an apple which was not the variety you wanted (this has happened to us twice) or if you just discover a variety which you much prefer to one that is growing in your garden or orchard.
With all kinds of grafting, the principles are the same. Cut the scion (preferred variety) wood in late winter, store somewhere cool and not too dry (in a polythene bag in the fridge is ideal) then carry out grafting operation in early spring when the new leaves are beginning to come out. Cambium to cambium contact is key.
Before attempting any sort of grafting, practice the technique with a small very sharp knife (I favour the Opinel number 6 stainless) on waste wood until you become reasonably good at shaping the pieces to fit together well. Good timing, clean cuts that fit, cleanliness, and correct wound sealing are key.