Let's say Pence and half the Cabinet invokes the 25th...

[Massacure]

8. All Pence has to do is deliver a letter signed by half the cabinet to Paul Ryan and Orin Hatch

Pence would deliver a letter to the Speaker of the House (Paul Ryan) and Senate President Pro Tempore (Orin Hatch) stating that Trump is unable to execute the duties of president. At that point, Pence would be acting as President.

Trump could then deliver a letter to the Speaker and President Pro Tempore that he is capable of acting as president. Trump would then resume control.

Pence then has a four day long window to retake the Pesidential powers from Trump (as long as 1/2 of the cabinet agrees). If Pence retakes the presidential powers, Congress has to meet within two days (if in session) or 21 days (if not in session) to decide the issue.

This is where things would get interesting, but my reading of the 25th amendment is that Pence keeps presidential power until one of three things happen:
1) The House votes and less than 2/3 of the Representatives vote to keep Pence in power
2) The Senate votes and less than 2/3 of the Senators vote to keep Pence in power.
3) Two (or 21) days pass and either the House or Senate has not acted.

The other interesting scenario I envision is what would happen if Trump were to fire cabinet members before Pence sends the second letter to Congress.