How do you make a character a part?

What you're looking for is a "morph" basically, or am I mistaken?

In order to set something as a character, you must have a model that contain part called 'Head' and 'Humanoid' object. That's the most minimal requirement, but this character won't be controllable. In order to control your character, you have to insert two additional parts, named 'Torso' and 'HumanoidRootPart'.

Now, the 'Torso' and 'Head' must be connected, in order for humanoid to live. The 'HumanoidRootPart' is the main part of your character, that will let you control it. I believe it has to be connected to Torso. In original characters, 'HumanoidRootPart' is welded together with 'Torso'.

Now, when you have a made custom character like that, you can change with code like this:

local char = game.Workspace.Character
game.Players.LocalPlayer.Character = char
game.Workspace.CurrentCamera.CameraSubject = char.Humanoid