Walnut burl veneer is inevitably both warped and also brittle.
If you're going to use it, it's worth making up or buying veneer flattening solution, wetting the veneer with it, and clamping it between two pieces of melamine or plywood with some packing paper to get it flat without destroying it. Change the paper a couple times as the veneer dries.
Once it's flattened, you can glue it down using a caul (one of the pieces of melamine you used to flatten it will work) to hold the veneer flat to your hopefully flat substrate while the glue dries, or you can hammer veneer it with hide glue.
Hammer veneering doesn't risk gluing your caul to your veneer, but it does require a warmer shop, and I wouldn't want to do e.g. a dining table that way. Melamine cauls usually pop free pretty readily if they do get glued to the veneer. If you don't have or want to buy melamine, a packing paper layer between the veneer and the caul will help, as will covering the caul with packing tape.
Ideally, you don't end up with a lot of glue bleeding through the veneer, but burl veneer tends to wick it through.
If you're going to use it, it's worth making up or buying veneer flattening solution, wetting the veneer with it, and clamping it between two pieces of melamine or plywood with some packing paper to get it flat without destroying it. Change the paper a couple times as the veneer dries.
Once it's flattened, you can glue it down using a caul (one of the pieces of melamine you used to flatten it will work) to hold the veneer flat to your hopefully flat substrate while the glue dries, or you can hammer veneer it with hide glue.
Hammer veneering doesn't risk gluing your caul to your veneer, but it does require a warmer shop, and I wouldn't want to do e.g. a dining table that way. Melamine cauls usually pop free pretty readily if they do get glued to the veneer. If you don't have or want to buy melamine, a packing paper layer between the veneer and the caul will help, as will covering the caul with packing tape.
Ideally, you don't end up with a lot of glue bleeding through the veneer, but burl veneer tends to wick it through.