EPT
1. Well, the first and most important part: Before you start, ask your peers (the people that use the template the most), how the template should look like. This point is essential, since all your work will be pointless, if noone uses your template because they don't like it.
2. Do a mockup: If you do the whole template, and then have to change anything, because it didn't work out as you thought, it will be much more painful, than just messing with a mockup. Messing with a mockup is way easier, because you don't have to change things at all the places.
3. If everything is set: Start coding. Now is the part, where you actually do the template. recreate your mockup, but with the parameters this time. Be sure to handle all relevant possible entries users could do.
4. Test: Test your template. Test a lot. Test until you're sure everything works as expected. You most likely won't be there to fix every use of the template where it was done wrong.
5. Write documentation: Your template may be awesome, but what does it matter, if noone knows how to use it? That's why you need to write documentation. The better your documentation is, the less problems users will have using your template, and that will possibly increase the use of the template. Noone wants to look in the code to see how a template works, and if the people don't understand it easily, they won't use it.
6. Start using your new template, and have fun. Most likely you just made contributing easier, because templates help people a lot with doing good looking pages.
Template:CrossTableBig
