First you want to just google forge for your version. I think there should be a download link (on the site just wait 5 seconds and click the thing in the.
In a new world, or an already existing one, add "WorldEdit" in Behaviour Packs and "WorldEdit [RES]" in Resource Packs. Also enable "Beta APIs" in Experimental.
On java, open the launcher, go to the skins tab > new skin > set the skin file to the one you want, the model size (slim or wide), and optionally name it >.