Here’s a handy feature for MS Paint that you might not know about. Did you know there’s an option for a grid.
That grid won’t show up on saved or printed images. It’s strictly there as a guide. Here’s what it looks like.

Click the View tab at the top of the page.

You’ll notice that Gridlines and Rulers are checked. A grid can be a very useful tool for cropping or creating an image, especially if you need straight lines. The rulers are measuring the size of the image in pixels.

Just check Gridlines to add a grid. You can also add rulers.

If you want to remove the lines, just uncheck.