The various layers in Robin's template are useful for different purposes. Obviously, the coarse and fine grid patterns are helpful for placing details with respect to the topography of the avatar body, and they give you some feel for the resolution of your texture when it's finally applied in 3D. The layers with match lines are extremely helpful in getting textures to line up properly across seams, and those with color bands can give you some clue about how to draw straight lines.
The avatar body has some regions where we generally try to avoid putting detail, either because you can't cram enough pixels into them to get decent resolution or because body movement or simple topographywill stretch the heck out of your texture. Try to stay away from joints -- shoulder blades, crotch, pecs. Expect major distortion on female breasts and male shoulders. Unfortunately, these trouble areas make a hash out of things we would all like to create ... wide lapels, fancy high collars, bikini bottoms and shorts that don't "bleed" down the leg, big logos on the backs of jackets .....
The best way to find out what works is to experiment. Make yourself a full body suit out of Robin's templates and wear it in world. Mess with your body shape to see what distorts, and take loads of photos of yourself in various stretching poses to see where the grids take the greatest punishment.
For some things like collars, the best solution may sometimes be to build prim attachments instead of drawing them into your base texture. That's the case when you want a stand-up collar, for example, or one with prominent lapels. The same is true for cuffs and some bodice components like bows and some kinds of trim.
Instead of trying to re-invent ways to handle a lot of the design challenges you are about to face, I suggest that you save yourself some trouble by reading through some of the SL-specific tutorials in the Tutorial sticky at the top of this forum. Try also visiting places like Natalia Zelmanov's blog to get a feel for really basic techniques that work in this particular virtual world. She's at

. If you are working on something and could use a critical eye or a helpful hint, post a photo in this forum and see who responds.
Welcome aboard!
EDIT: Incidentally, you may want to search the archives of this forum to see what other people have already done with polo shirts. For example ....
