I researched grafting cherries pretty extensively before I attempted to graft my first cherries. Cleft graft seems to be what most people had success with. I cleft grafted 10 cherries scions both sweet and tart onto Mazard root stocks. I had 100% takers which really surpised me. If the scion is smaller diameter than the rootstock push the scion over to one side to align the cambium.
More specifics are here —> Grafting Cherry