How long does a transfer usually take?
What steps do all parties need to make?

Go to https://knowledgebase.ruralinfo.net/ and search 'mutual transfer.' This will get you started.

The form is real simple. You sign it and get your station manager/postmaster to sign and then make a copy for yourself. Send the original to the other carrier, same signatures are required from them. They send the original back to you and you submit thru your supervisor to HR. It can take as long as three months when multiple transfer are to occur. If its just a two-way, normally six weeks. You will both start on a Saturday, first day of a pay period. The HRs in both districts have to sync the start date with each other, one reason it takes so long. You will get about a weeks notice on you start date.