If this was a pencil sized rootstock and scion you might consider a standard Italian grafting tool https://www.garrettwade.com/professional-italian-grafting-tools-gp.html?SCID=W4011000&mkwid=stufgSQfw&crid=253562544928&mp_kw=&mp_mt=&pdv=m&gclid=EAIaIQobChMIiZHf-_z72wIVk4RpCh2LqAPREAQYAiABEgJ5hvD_BwE. If it’s a larger diameter tree this thread will help Top working Pears weather permitting. The cambium layers must match up and whatever your grafting must be compatible with what your grafting it to. Temperatures need to be on the cooler side and consistent. Temperature fluctuation can impact callusing if it’s to hot or to cold. If callusing does not occur the graft does not take. The cambium layer is inside the bark layer which can be seen clearly here in these photos 
From this thread Che, mulberry, osage orange, fig grafting. This method is called rind aka bark grafting and you can see the small scion is inserted and directly in contact with the cambium layer.
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