Try these steps just to get a very basic idea.....
I'm assuming you've downloaded the templates. The lines you see on them are to help you line up seams and straps, etc. because when you bring the texture you make into SL you're taking a one dimensional image and placing it around a 3 dimensional frame. There are going to be some mismatches in seams and things and as you practice you'll find its necessary to redo things often.
Open the upper body template
You are going to create a simple tube top
Open a texture you like, say a nice floral print like this one ..
http://www-scf.usc.edu/~dacey/images/red%20fabric.jpgLook under tools, find edit and copy the floral print
Switch back to the upper body template and paste the copied image on to it
Move it into position over the frontside of the body image, lining it up to cover the chest area
Paste it again onto the template and line it up over the backside of the body image.
(doesn't need to be even for this initial exercise but it's a good time to go ahead and practice using the lines to line up the two squares. You'll find the square you paint on the backside of the body image will sit slightly higher than the square you paint on the frontside of the body image.)
Now, go to the side of your screen and you'll see a box labeled "layers". It will show all of the layers of the template you are using. Right now most of them should be closed except for the two floral images and the upper body image. Click on the little eye to close the upper body image and anything else that might be opened, until the only two layers still open are the two floral squares.
Go to your tool bar at the top and find "layers" there. Scroll down to "merge layers". The two floral layers will merge into one.
Now go to your side tool bar and choose "magic wand". Use the magic wand to click on the window showing your two floral squares. This will cause them to be outlined in little black lines.
Go up to selections, scroll down and choose "invert selection". This will cause the little black lines to move around only your floral squares and nowhere else.
Go back to edit, scroll down and copy the squares surrounded by the little black lines.
Go to File and scroll down to New. A window will open offering you options... choose select a new rastar image.
When the rastar image opens you'll see another window that looks like a checkerboard similar to what you've been working on.
Select paste and watch the two floral squares appear now on the new window.
Use your "move" tool from your side tool bar above where you found the magic wand, to move the floral squares exactly into the same position so that they match the ones in your original template window.
Almost done....
Go to layers in your top tool bar, scroll down to "make a new mask layer"... choose "from image"
Use layers in your top tool bar again and scroll down to "Load/save Mask Layer" and choose "save mask to alpha"
Now go to File, select "save as"
Name your image (ex: tube top)
Save it as a targa.tga image
Close psp
Go into SL
upload image

Good luck... I know it all sounds very very complex but do it a couple of times and you'll be a professional designer in no time.