Writing your own contracts is perfectly possible, and legal. But it's also an incredibly bad idea. There's two reasons for this: Property law is complicated. Because it's such a fundamental part of legislation, it's often lots and lots of different laws layered on top of each other.
Contract Assignment. (a) A contract assignment must be made as part of an ownership change, a change in tax status, or a transfer from one legal entity to another through a legal process. No assignment is effective until approved, in writing, by the Texas Department of Human Services (DHS).