Iceblossom

As said by NativeTexan, when using templates made of plastic keep in mind they are not rulers! Position your ruler to your template or draw around like Ckcowl suggests.

I buy the large sheets of gridded plastic on a regular basis, it's a good thing to use Joann coupons for. Be warned that they are not heat proof and will melt, distort or do other bad things even if you are just trying to iron them smooth on warm... I understand there are heat resistant products but I'm cheap and haven't found them yet.

But speaking of cheap, I often use the pieces of cardboard that often come inside commercial fat quarters for templates and keep a couple in my sewing kit to have them handy. Usually it's a good size and the cardboard comes in different weights, sometimes stiff enough to trust cutting against. But be warned, with the template plastic or cardboard sooner or later you will cut into the template ruining both it and the fabric underneath. With my vision issues I have problems with rulers, I make a lot of templates or what I call "snubs" which are shapes to trim with.

Edit: As for the markings, I will either draw it out on the grid lines or trace over it for a shape. When using cardboard I draw out/copy the shape on graph paper and tape it down to the cardboard. As I trim the shape, I cover each cut edge with clear tape folded over the paper from front to the cardboard on the back, it seals everything nicely and gives a nice edge to work with. One of my cutters is designated for craft use including cutting paper and batting.

Edit 2: When drawing on the plastic make sure you are using something that works enough. Sharpies are usually good but sometimes smear a bit, especially at first. Some pens will smear and get on your fabric! If your vision is better than mine you can kind of scratch in the line with a ball point pen even if the ink doesn't come through. I'm going to be doing a OBW/Stack and Whack and in that case I will draw lines on my template on where I am placing my template, pencil can be erased or you can carefully work with ink that will wash off.

Among other odds and ends in my sewing kit is a roll of 1/4" tape. One use for that is marking off the seam allowances in templates while leaving the center clear. Also use on rulers and a bunch of other ways along with my blue tape.