Personally, I think British English is brilliant. Unless you're specifically targeting American readers, or trying to write dialogue spoken by Americans, I'd suggest you write it your way. I'm pretty sure Rowling didn't care if she said "boot" or "trunk" when referring to an automobile (there was at least one car in there from what I recall).

If you are trying to write dialogue spoken by Americans, I'd still suggest you just do it your way and later ask one of us Yanks to help edit it. The important thing is to get it out of your head and onto paper (or the virtual equivalent thereof).

Heck, even the Doctor Who writers don't bother to get American English right sometimes.

Two cultures separated by a common language, indeed.

Anyway, the idea is to spend *at least* 15 minutes doing the exercise. More is good. I think Battywyn🎶Prep! said 15 minutes minimum to emphasize that almost anyone can find the time to do this every day. I routinely spend hours thinking about it and then way more than 15 minutes writing.