If you make something over and over again you want a template or maybe a brush. This tut is about what I have learned when making templates for Gimp.
Start off with a blank canvas, with a transparent background, 200-300 dpi (your choice).
The size of your canvas should be determined by what you are needing and how big they need to be individually and with enough room to use the guillotine tool. Don't forget that your completed image may contain extra elements like drop shadows and ragged edges (after guillotine is used, the canvas can be enlarged, however).
If you are doing a sheet of goodies (like slides, buttons, eyelets, etc), I use a 3000x3000 canvas, but I may use a second sheet so they are 1. at least 600 pixels in size, and 2. far enough apart for design effects and finally 3. easily cut from the sheet.
If you are doing a sheet of alphas, I use a sheet 2000 to 3600 pixels square. Don't try to put the whole alphabet on a single sheet. It's not worth it. The guillotine tool will make a lot of new images. Do you want 38 open windows? Just keep track of your steps so you can repeat them on the next sheet.** Make and save all of your needed templates before you start making your project. Close unused templates, you can open them later (cuts down on system resources).
When organizing the letters or items on your canvas, try to keep your horizontal and vertical lines clear. Do this by dragging your mouse from the side or top ruler into the position you need it (a dotted blue line will appear). If you want to be exact, then you can add guides with the Image>Add Guides tool from the canvas view and put in a specific coordinate.
About guides: These guides are awesome! First of all, they help you keep your images organized. Then these guides can be used with the Image>Transform>Guillotine filter to cut apart your image. The guides can be used to cut the image apart even if you haven't merged your layers, in case you want to work on the images with layers further, but separately from the other items on the template. Finally, these guides can be saved if you save your template as an xcf file, which will also keep your layers inact (just don't merge before you save).
About guillotine: Let's say you made an alpha and you are done with all editing. Merge all visible layers. Then save the whole sheet as "myalphas.png" in the folder you want your alphas saved in (this saves soooo much time later). Now, use your guillotine tool. Your image will now be a bunch of images that are called something like "myalphas1-1.png" as the tool renames the new images so none are named the same. You shouldn't need to do anything with the images that just popped up. I have a great shortcut for you... don't panic! Click the red x to close your image. A box will pop up asking you if you want to save changes, click Alt-S and it will save. I don't like the number system so I rename my alphas and elements in batches by using the renames series tool in my image viewer ACDSee. I love, love that program.
Notes about patterns and gradients: How you have your templates will determine how your gradients look. If you have a single line of letters, lets say A-E, and you use Script-Fu>Alpha to Logo>Glossy, with a gradient, then you will have that gradient from top to bottom on A-E. If you miss a color, then there's too much room on top or bottom of the letter (empty canvas) so the gradient slips off the letter. Reducing this may interfere with the drop shadow. Don't worry. You can delete that and the background layer later and redo the shadow. I recommend shadow settings of 2/2/3/60. It gives the dimension without being overbearing. I also don't always use a straight black on my d'shads. I may use navy, brown or forest green. If you have a sheet with let's say five letters across and four rows down, and let's say the gradient is a rainbow, your A will be mostly reds and oranges, with the last row being blues and purples.
You have to keep an eye on your patterns too. I have some that are not seamless tiles. If the pattern is 600x600 pixels, I make sure my guillotine lines are on the 600, 1200, 1800 and 2400 (vertical and horizontal) spots so my image doesn't have that seam. Sometimes making an image seamless in Gimp makes it so distorted you won't like it. I am trying out a program now and maybe I'll use that until the Gimp Geeks can find a good filter that doesn't wash out the image like it does now.
**I would have your main preferences to max out the limits for the undo tool. You will not believe how many little things you do and then want to go back, only to find out every little brush click chucked your previous steps in the "trash" because you had it to keep only "20" undo levels. Plus you may want to go back to a certain spot and then change up. For instance, I will take a shape brush, make a white background layer, add a drop shadow, make a top colored layer and a drop shadow, then bucket fill. I may add canvas, bevel or other effects. After I merge and save, I go back to a certain spot and start over again with another color or pattern in my scrapkit's palette for the same frame but in a different "flavor".