Is there a free photoshop/paint program with clear instructions?

I’m a painter, and I work from photos on my computer—just to get me started, blocking out the stuff I want to put where on the canvas, and I find that photos help me get the proportions right.

That is, I don’t do most of my painting directly from the photos—they just help me get started, and I sometimes refer back to them later on if I don’t like what I’ve painted. Most of the painting is freehand, but I do rely on photos, after I crop them, blow them up, and otherwise distort them, so I’d like to find a good program to display on my monitor as I begin to paint. In particular, I need one that projects a grid so I can be sure that my proportions are precise.

I’ve got Paint 3D on my laptop, but I find it difficult to use (can’t find a grid-maker, for one thing) and the online tutorials don’t help very much. (Among much else, they instruct me to find “view” on the ribbon, and as far as I can tell, there is no “view” tab on the ribbon.) Is there any photo program (preferably a free one) that comes with clear instructions?

Krita has an online manual including tutorials. You can search for “grid” on that page and it explains it, e.g., Grids and Guides Docker — Krita Manual 5.0.0 documentation
Using reference images: Reference Images Tool — Krita Manual 5.0.0 documentation

I like PixBuilder.

GIMP

I don’t know if paint.net will do what you need, but it seems to be a fairly robust photo/creative type of program.

Maybe I’m asking for the sun, the moon, and the stars here, but I think I’m asking for something relatively simple: all I want is a photo program that will allow me to take my own photos and print them out with a grid (preferably changeable) over them, so I can tell if I’m making something in the photo too big or too small in my canvas.

But every photo program that I use either doesn’t have a grid, or the grid is too tiny or it won’t let me print it at the size I want. Just now, I tried to print out a photo that shows up on my monitor as taking up approximately the middle ninth of a nine-box page–when I try to expand the size to the full page, it prints out at the original size, too small for me to use. I’d like to edit the photo so that it’s the size I want.

Have you looked at XNView, it is free and can show a grid that you can customize to some extend.
However, it will not print the grid.
What you would need to do is get a screenshot of your picture with the grid on.
You can use the tools " Snipping Tool" or “Snip & Sketch” to get the screenshots that you need. Once you have that you can print the resultant picture with the program of your choice.
Of course, you can use any other program that you already have to generate the grid on screen.

If you are just compositing images, GIMP has layers, and if you go to the Image Properties you can check the print size/resolution or change it from the image menu.

I like GIMP. It’s free, powerful, and has a lot of features. However, the OP is looking for something that is easy and intuitive to use, and has clear instructions. Unless things have changed dramatically since I installed the program, that’s not GIMP.

Paint.Net (already mentioned upthread) is much more intuitive and easy to use. You’ll have to install a plugin to do grids, but once you get past that the rest is easy and intuitive.

GIMP can also do grids. I can’t remember if it comes with the program or if you have to install a plugin.

Most software that’s worth bothering with should have a ruler, or at least image information which will give you the size of the photo. Some software may have the ruler hidden so you may need to do something like; View>Rulers.
It would also help to have a basic knowledge of image resolution. Screen resolution and print resolution are 2 different things and will affect the size of your print. Google it. It’s really quite simple but some have trouble grasping it.
If you want software that’s worthwhile there’s bound to be a bit of a learning curve.