Question on Custom Character Sheets when I'm not GM

I'm part of a long-running campaign which uses a homebrew system, and would like to design a custom character sheet for the game, though I am not the GM.  Is there a way that this could be accomplished as the GM is not a pro member?  Would I need to subscribe to pro twice (once for me, once for him), write the character sheet and then have him implement it for the game? Apologies if the question has been answered previously.  I did not see it.

Or you could (once you have pro and have created the sheet) create a game with that sheet, invite the person that wants to run a game with it, and promote that person to GM.

Got it.  Thanks!  In that case, I presume that he could temporarily promote me to GM and I would implement and then transfer it back.

No.  Features available in game are determined by the game creator.  For someone else to be the game creator and have a custom sheet, that person needs to have the pro account as well.