Also can you graft sour cherry onto sweet, or vice versa

>>Is fruit scion grafting hard or easy?<<

It is easy ones you know how...takes some practice and maybe a couple of years...the more you do it the better the takes. Yes, cherries graft to each other.

It's pretty easy I had good success first trying. Pomme fruit is most forgiving. I grafted sweet cherries had success first try the type of graft where you put scion into side of bark on mother tree, not sure what it's called but went 7 for 8 doing it. Watch a couple you tube videos.