Supplies: Helix T-Square, Cutting Mat, Krylon Black/Matte Cover Maxx (optional, for frames to match), frame(s), poster board (I got mine at Dollar Tree, two for one dollar)
I measure the whole piece I'll be cutting out a "mat" for and cut that out.
And measure how much I want the mat to cover the art piece. I didn't want the mat to cover "moon" on my poster (it's from my first telescope) so it's a bit small and imbalanced for the frame, but at 50 cents for the "mat" I can change it later if it starts to drive my pedantic designer-brain crazy. (Not likely.) The mat ended up being about 1.5 to 2 inches.
After measuring the mat size I cut along the guidelines I cut out where the picture will show, running my x-acto knife along the t-square.
Then I see how it looks in the frame! To make all of my hodge-podge old frames match I gave them a coat of Krylon Black/Matte Cover Maxx Ultimate Coverage which has a nice satiny finish that makes it look like the frames were always black.
This DIY is also a good option for oddly-shaped pieces there are no pre-cut mats available for. The result is not as gorgeously polished as getting a piece professionally framed, but it's a nice way to get your favorite collection or art pieces looking nice on the wall. You can always get it professionally done later if you have the money and want to support the frame shop.
Do you have any hacks for getting collections on the walls? I've been playing around with ideas and have a few more to share soon!