I handled all of the writing of TY notes after our inadvertent engagement party (that's another story) because while FI is a genius when it comes to things like science and math and building stuff, his handwriting is illegible and he makes a lot of grammatical errors. He did, however, sit with me while I wrote all of them and helped me come up with a individual note for each one. I expect this is the same way we'll do the TY notes for the wedding.
ETA: He also bought stamps and mailed all of them out.