If two states want to become one state how may they do that?

Interesting question. Both states would have to amend their constitutions to be the same and to include the other state's counties and other entities, not to mention combining their state agencies in all three branches. They would lose two senators, although how that would happen may ultimately be up to the US Senate. The united states would share representatives, and undoubtedly there would be some interesting redistricting issues. Again, the House of Representatives would ultimately have to decide to accept the proposed representatives. Special elections would probably have to be held for all now-combined state and federal offices.

I doubt any two states separated geographically would be able to merge successfully, and I doubt any two states would want to merge.