If you go on YouTube and do a search for Mango Veneer or Mango cleft graft, you will get several very informative videos. Truly Tropical has great videos on YouTube as well.
Graft when it’s warm and there is active growth on the rootstock and prep your scions before you remove them from the donor tree. Practice in the meantime with random plants that have a similar hardness of wood as Mango. I like to graft green scion onto green rootstock branches whenever possible.
Simon