When I'm thinking about what to tell here about making your own clothing textures, a lot of things come to mind... Let me first give you some global information about how I start a design (and in a next item how finish it). After that I'll cover more detailed stuff. If you want to know something specific, let me know! When you start making clothing for second life there are numerous different things you need to know, or they are at least handy to know. Here are a few...
Are you going to wear it yourself, or are you going to sell it? Or both? If you wear it yourself and there are some mistakes in it, no problem... If you are going to sell it, people in most cases will not tolerate mistakes (depending on the price too...). They may not complain, but will never return to your store for new items...
What will you make? The better you know this the better you can work on a collection of items that work together (so you can mix and match!) and if you want to sell it people will know that to expect from you. You can always change if your choice isn't working out for you.
How do I make a typical design from scratch? It is in most cases done in three steps:
(have a good idea in your head and) sketch it on paper
draw the design (in lines), this is the base for your design
fill in the lines, add all the details, save and upload it
Sketching on paper
All my designs are mostly hand drawn. this means I draw the lines myself (doh...). It is also possible to use a picture from a piece of clothing or fabric and 'paste' it onto your avatar (photo sourcing), this can sometimes look spectacular, and sometimes like crap :) On the left you see Ellen wearing a never sold design (the only design) in which I used a picture of a blouse / shirt. (the original picture is from my rl gf who posed for the picture in one of my white shirts). The rest is hand drawn... A little note to that is that some of the details on my items are (part of) photo's: small parts of belts and zippers.
When you have all these kind of figured out you can start without even touching your computer... The best way to get your ideas into shape is by drawing them on paper. You can easily correct things or start over... and more important: complicated software won't break your concentration. When I started I've made that mistake so many times... ended up with no work done, forgot what I actually wanted to make in the first place and completely frustrated...
I know it can be hard to draw a human figure before you draw your design on it. So what you can do is put your avatar on a poser, against a white background and make a good frontal snapshot (and one of the back too) and print both images on a piece of paper. This way you only need to draw your idea over it.
Then there are some limitations in designing clothing for an avatar, most of them you need to find out while designing as I did in my first designs (which took me weeks and a lot of L$ while uploading the textures). Lines and textures sometimes just don't look the same on an avatar as when you were drawing them in your texture. Textures get wrapped around the avatar body en there are places where the texture is stretched (a lot). (how this works can be seen in this blog of SL Natalia) Typical area's that cause a lot of problems are the side of the boobs / under the armpits. Boobs in general (the bigger, the more stretch, the more blurred the detail will look...) and between the legs of an avatar... So basically don't put (too much) detail in those places. On other places a straight line can show strange twists and the only thing to change that is to redraw your design.
Next item, more stuff about the use of programs and how I finish a design!