grow both tomatoe plant and potatoe plant to the transpant stage harden them off let them grow side by side out doors for one week make a one or 2 inch cut in the skin of each the plants where they will make contact then tape them together so no cut is exposed . Next let them fully recover . Then gradually cut back the potatoe leaves a few at a time until there are none ,let them return to good health check the graft to make sure the 2 plants have fused ( grown together as one ) if they have ,and the graft and tomatoe plant are both healthy and strong you can then amputate the potatoe branch just above the graft . Then let it grow until it is healthy strong stable and growing when the tomatoe is growing well and healed completely then you can cut off thr tomatoe roots . Then when it heals completely you have a tompotatoe plant ! The harvests you get are usually inferior to growng the 2 crops separatly . Good Luck!