First, you are going to need to open NovaSkin.
NovaSkin:http://minecraft.novaskin.me/

Then, find a skin you like on PMC and click the download button on the right below the change button. It will then download on to your computer. I would recommend saving it to your desktop so it's easier to find. Go up to the top left of the page and right below the gallery button should be an open button with a little folder next to it. Once again, go up to the top left of the screen on NovaSkin and click on the Pose tab. Then you can pose the skin by sliding the button across the bars on all of the parts, or you can choose a premade pose. If you want, click the hold right, or hold left buttons to make your character appear to hold an item :) Be sure to credit the original maker of the skin :) Another blog will be coming tomorrow for a tutorial on having more than one skin on the same picture, like the image for my last blog :)