My comments are specific to black walnut. Walnut is among the most difficult species to graft. By properly collecting and storing scionwood and making good quality grafts, I get 90% success.
The best takes are with one year old scionwood that has a large terminal bud. The terminal bud will trigger growth even when other conditions are unfavorable. Proper size and condition scionwood is best collected from vertical growth near the top of the tree. I often cut a limb off the tree so I can get the vertical growth at the tips of the branches. Don’t collect scionwood after several days of sub-freeing temperatures. Wait until at least 3 days have passed with temps above freeing during the day and no lower than 28F at night. Cut scionwood with a couple of inches of 2 year old wood at the base of the stick. This gives plenty of room to whittle to make good grafts and leaves extra moisture in the wood. Two year old wood also makes an excellent base to attach the scion if using any type inlay graft. I store walnut scionwood in a refrigerator crisper drawer in gallon ziploc bags with moist promix BX, a peat based seed start mix. Promix is sterilized before packaging and tends to prevent fungal growth during storage.
The only other suggestion I’ll make is to learn to make inlay side grafts. Walnut tends to bleed if you cut the top off so I make an inlay graft into the side of the tree. Cut the scion with about 3 to 4 inches of wood to contact the stem of the rootstock. Attach the scion tightly using a couple of 19 ga. wire nails. (nails are sold by beekeeping suppliers as 1 1/4 inch frame nails) After two weeks, I cut the top out of the tree a couple of feet above the graft. This forces the graft into growth and prevents flooding under the scion. As the graft grows, I tie it to the stub of the tree for support. It is normal to get 5 feet of new growth from a graft to a 1 or 2 inch diameter established rootstock.