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.
Step 2: Engrave the Image.
In the trace step we're going to lightly engrave all the lines of the template. The lines should trace the template 'perfectly' but don't have to be a master piece in terms of depth and homogeneity.
This is actually the hardest part. Because your glass will be of some thickness, depending on the angle you look at it, the template will appear in other places of the surface of the glass! KEEP THIS IN MIND! and rotate your glass with every letter or stroke. Otherwise the engraved image will be distorted. One simple trick is engraving some reference points first like the belly buttons, the eyes or other small dots in the picture. Engrave them while looking directly at them!. Then line those points up with the underlaying template before engraving the other lines around it.
In the finishing step You take out the template. FIRST: Make sure you've engraved all lines, then remove the paper end replace it with a dark piece of something... black socks in my case, anything dark that will make the engraving more visible is good.
Now these lines engraved in the glass are your template. Now you can make all lines the same depth, fill in all the large black fields and mask all the small mistakes :)
Use pliers to change the 'drill' bits.
Admire your handiwork! :D
I excuse myself for any spelling mistake that might occurred, English is not my native language. But please correct me if I typed something wrong, we're all here to learn.