You can do it the easy way (if you have internet) and just upload it through your preferences. Or if you don't have internet you can do it the manual (hard) way. To do this open your Minecraft.jar file with winrar and apply the character skin by replacing the "char" file with it (note: your skin must also be called "char"). To apply a an overall texture skin (grass, rocks, ets) just stick everything in as is. I hope this helped : )