For this instructable, You'll still need some engraving tool like a Dremel or something like that... (I have a Proxxon for years and years now, still working like a charm)
But these devices are a tiny bit less expensive and easier to handle than what I read about the other methods.
Step 1: Step one, finding and preparing the image
So making a template:
If your template only consists of letters I mostly use a dark background with gray letters on top. The letters have a fine white contour line. (see image)
This form is actually the easiest by my opinion. You can see where the glass is already engraved thanks to the gray, the black makes errors easier to spot and to prevent.
But that 'perfect' form is not that easy when it comes to images, unless you are a photo-shop master and have a lot of time. So just black letters on a white background should do. But it's harder to see where you've already been, it all comes down to the perspective. (more on that in the next step)
The template used on this project is also attached as image.
Print your template centered on a piece of paper. Fit it in your glass/jar or lay in underneath a flat piece of glass (depends on what your engraving).
fold the paper, or shorten it so the image is in the optimal position in/under your piece of glass. Tape it down, or immobilize it in a way so it won't move during the engraving.