When I'm doing any fussy cutting, I make a "window" the size I need out of a file folder or any light weight cardboard.
By window I mean drawing the square on the cardboard, cutting it out and using the window or hole to lay over the part that I want. I draw around the inside of it with chalk or pen, depending on the color. Then I cut it out with a straight ruler. My fabrics always look like Swiss cheese. Works for me.

I do it the same way as rocklady. I cut my "window" out of cereal box cardboard. Then I use the plain side up to move around and frame my square. I use a chalk pencil or one of the Frixon pens. Then I lay my ruler along the frame outline and cut my "window". The Swiss cheese fabric can be cut into squares or strips. I use the strips to make string quilts. The squares go into scrappy quilts. If you can find a hardware that will cut plexiglass, that works also. You can get different sizes very cheaply.

I also make a "window" template out of template plastic. Like to be able to see through it.

I would make a template with template plastic and a 9" square, and a 1-2" border of the plastic, this way you can move it around until you get the fussy cut that you want.

I have this pattern to and will be following to see what good advice you get.

I use the picture frame or a frame matt out or the glass from a frame they came in so many sizes - shapes.
Oval, rectangle, square etc.

I use the picture frame or a frame matt out or the glass from a frame they came in so many sizes - shapes.
Oval, rectangle, square etc. True you can not get any size, sometimes I cut clear plastic.

I would either be tempted to invest in a 12 1/2" square ruler or get some stiffish clear plastic and mark a 9" square on it, check markings are correct then cut either old rotary blade or scissors.label square. Just be very careful when cutting round as it is thin but being clear it helps with fussy cutting.
Good luck.

A little of the red or green frame will be left with the fussy cut but that ends up in the seam allowance. Some of it even showed up in the quilt but you cannot tell it when the blocks are sewn together.

Very good hint! I will try this the next time!