Background: I have a full-color sign I use for advertising my party, with the space for the room number left blank. The usual approach is to fill in the room number with a black marker. Or wait until the room number's been assigned, and modify the color graphic image to include it. (Or print out the room number on transparent labels.) Sometimes it makes sense to print the color pages and then over-print the room number later. While this could be done by creating your sign in "layers" and then just print the room-number layer, that assumes that you have full access to your paint software and the same printer which might not always be the case. So I came up with this system. If there's interest, I may make the program available as a Win32 or OS/X executable, or perhaps have it available on a website with a password to generate the grids?
Use the first grid, save it to your computer and print it with some other program. This grid was generated at a 275x400 pixels size with 25-pixel gridline spacing, so it'll print pretty small if you print it from your browser.
The program could be run to generate a larger size image (for example, one the same dimensions as your sign) -- here's an example that's 500x650 pixels and another at 600x800, one of them should fill most of your printer's page. (And as we'll see below, the important thing is for the grid to cover the part of the sign where you want to drop in a room number or other text.) So if you were using a larger grid, you could use your browser and turn off the headers and footers before you print. We're just using a smaller size to make it easier to explain how it works.
Line up the template with your party sign, and shade in the squares where you'd like your room number to be overprinted. Figure out what would be the upper-left corner, and what would be the lower-right corner, when you turn the sign and write in your number.
Locate the grid squares that correspond to the upper left and lower right corners of the block you'll want to put your overlaid text in. (The program I've written can deal with different size grids; I've created one that's 275x400 pixels because that should print fine on most computers, even from your browser.)
Then all you have to do is feed in the coordinates along with the text you want to print, and print the output (such as the diagram above) the same way you printed the template.
You can specify rotated text simply by identifying the "upper left" and "lower right" corners of how it'll be displayed. These samples have the grid showing, and a shaded block.
The program will automatically scale up to the largest point size it can fit, and will center the text (such as a room number) in the block. You can pick an alternate font.
There's an option to allow you to re-use an image and add more text. In this example, I've put 632 in spaces L2-L4.
As you can see, the grid and shading is optional; I just had them turned on for demonstration purposes.
And as long as you print the grid using the same program, settings, and printer as the results, they'll line up perfectly.