These are Canon art works for what is going on, and this is just an explanation for the other parts I missed.

*SPOILER WARNING*

All drawn by ~me~

The year which they are originally in is 2015.

Stan and Ford are Mabel and Dipper's grandpa's.

So here is Mabel's outfit for different timelines:

That's all canon. Also, here is a part where Mabel almost encounters her past self from 'Sock Opera'

I will do more character doodles later on.

Anyway, the 1st journal is in 1647, where Ford is.

Dipper, Mabel and Stan disguise themselves as that timeline. Mabel's outfit is drawn there.

*SPOILER* — Ford nearly gets execution due to a crime he caused by trying to stay alive. Can't tell you how they get him out of it..

But in the depths of the city.. someone is watching them..

Anyway, they save Ford, blah blah.

So the 2nd book is in cowboy/girl times, 1815.

Mabel's outfit is shown there. Her old one gets burnt, but she puts the spare one on (Blue one in picture).

Dipper, Stan and Ford disguise themselves as cowboys too.

*SPOILER*— Ford and Dipper do a 2v1 shoot out with someone, who is the 'unbeatable' player. (REAL GUNS!) They do this in order to get the book. Can't tell you if they win or not..

Once again, someone is watching them..

The 3rd and final book is somewhere during the time of Sock Opera, in 2014.

Turns out Bill Cipher has it. He posses old Dipper once again, and the entire episode plays, except the new Ford, Dipper and Mabel are trying to stop Bipper.

Especially Dipper.

Stan is off doing god knows what, trying to blend in as his past self with past Soos and past Wendy and the past others.

*SPOILER* — Mabel has a situation where she nearly bumps into her past self whilst chasing Bipper. (Happened some time offscreen, don't ask when lmao)

(Also, you guys realise that 1 plus 2 plus 3 is 6? And there's 6 fingers?)

Just something I realised.

Anyway, that's all I can say. Other canon outfits will appear later in the book.