Alright, so for this tutorial, you can make your ingame minecraft skin an HD skin without mods.

FIRST OFF, you will need a texture pack, sorry, but i'm not sure if there is even another way to do it without one

So after you properly install that, head back to the .minecraft folder (it should be in "AppData"):

Open it right up, and select your resource pack. Follow it through until you get to the textures:

Click on that:

And delete/backup that Steve thing. But make him go away

Now, get your HD skin, and rename it steve (it must be exact), and paste it into that folder.

Run Minecraft offline, and your skin will be HD