So, I actually tend to start with a shoebox to collect things so they stay in good condition and don't get lost. Then it's a matter of what techniques you know and what design elements catch your eye. I think it's really great to learn techniques from YouTube. That's how you learn things like how easy it is to glue an envelope onto a page and keep memorabilia inside that envelope (possibly those clues, or the letters). You can also make pockets by cutting a piece of paper and taping it along the edges you want closed. Using washi tape lets you choose another aesthetic element, but clear Scotch tape works just fine.

It's really what you make of it and what you're comfortable with. You can use your own handwriting or watch videos on how people do "lettering" or use stencils. If you remember what happened in each session, you might be able to "journal" it from the point of view of your character. Or it might be just a record of how things went and what was used, without necessarily needing to make it seem like something your character was writing.

It's probably a good idea to think about what you'd like your project to be before you start. It's okay to just keep everything together and spend some time thinking it out and watching videos for ideas or looking at people's blogs. It's okay to start slow and build over time. This can be a lot, but it doesn't have to be. It's about making something you like, giving yourself a chance to learn new skills, and enjoying the process as much as possible.