How do you weld models, or are you even able to?

No, you cannot weld models themselves. You should first weld all the parts in the model together, then weld one part in the model to the character's head.

Here's a weld script I made a while back, feel free to use and manipulate it to make it work for you. Please leave an upvote if I helped! :)

local prev
local parts = script.Parent:GetChildren()
for i = 1,#parts do
    if ((parts[i].className == "Part") or (parts[i].className == "Handle") or (parts[i].className == "UnionOperation"))  then
        if (prev ~= nil) then
            local weld = Instance.new("Weld")
            weld.Part0 = prev
            weld.Part1 = parts[i]
            weld.C0 = prev.CFrame:inverse()
            weld.C1 = parts[i].CFrame:inverse()
            weld.Parent = prev
            parts[i].Anchored = false
        end
        prev = parts[i]
    end
end
for _,item in pairs(script.Parent:GetChildren()) do
    if item:IsA("UnionOperation") or item:IsA("Part") then
        item.Anchored = false
    end
end