One more note, a grid is super helpful (indispensable, really) but I find that importing a line grid, like the one Dopan provided, can sometimes mix up your tile placement, because the lines themselves have to have thickness, even if only one pixel. You can usually set a grid as guidelines in a graphics editing program like Photoshop or GIMP,and those won't have that problem.
But for my gridded needs, I make color blocks the size I want (in your case, 48px square) and copy-paste them in a checkerboard pattern. I've done that for years, in a variety of different programs. Hide the checkered background layer or flood it with the color you'll be setting as transparent, and you'll be good to go.
(Sorry I can't provide more detailed instructions, I'm on mobile and don't have a graphics program handy to check the finer points if setting up a grid.)