I have three forms of templates for fussy cutting:
1) Purchased rulers that I have added 1/4 inch tape around the edge to represent the seam allowances
2) Plexiglass squares that I had cut at the hardware store. I added 1/4 inch tape to represent the seam allowances.
3) Template plastic that I cut into squares and added the tape.
The tape frames what you will see and helps with keep the templates in place. I have also added sandpaper dots to help keep the templates in place on the fabric.
One could also draw the seam line on the templates and use that.
Some prints work better in different sized squares. I cut some of my leftovers from cutting squares into rectangles and a couple of very neat quilts have been made using the rectangles.
I have used a chalk type pencil to draw around the templates to check for optimal placement. Or that darker pencil that also washed out (forget the name of it) Sometimes I get a better idea of where to cut after some preliminary placements.