This is nor really part of the Essentials. This is very basic in RPG Maker XP: adding graphics. Nothing more than that.
Anyway, what you have to do is create the HGSS style character sprites (either draw them in some image editing software or use one that was already done), then put it into the Characters folder in your project and use them in your game. There is nothing specific about the sizes or anything. Try reading a bit about how images should be diveded and so on to fit the RPG Maker XP style, this is really simple and basic.
Same thing with HGSS overworld tileset, just create a tileset image, then add the ripped graphics.