Help me weld this to my torso?

Just change the Models's PrimatryPart CFrame to your torso. Assuming that the model is about the size of the torso. To change the CFrame of a model do :SetPrimaryPartCFrame()

local Player = game.Players.LocalPlayer
local Mouse = Player:GetMouse()
local debounce = false

Mouse.KeyDown:connect(function(key)
if key == "p" and not debounce then
debounce = true
local RinkakuModel = game.ReplicatedStorage.WorkspaceRinkakuModel:Clone() --Assuming the model is now in ReplicatedStorage
local   w = Instance.new('Weld')
w.Part0 = RinkakuModel.PrimaryPart
w.Part1 = Player.Character.Torso
w.Parent = RinkakuModel.PrimaryPart
w.C0 = CFrame.new(0, 0, 0)
RinkakuModel.Parent = Player.Character
RinkakuModel:SetPrimaryPartCFrame(Player.Character.Torso.CFrame*CFrame.Angles(math.rad(Player.Character.Torso.Rotation.X),math.rad(Player.Character.Torso.Rotation.Y),math.rad(Player.Character.Torso.Rotation.Z)))
RinkakuModel:MakeJoints()
RinkakuModel.Name = Player.Name
end
end)

Hope it helps!

Game Script in ServerScript Service

game.ReplicatedStorage:FindFirstChild("RinkakuModel").PrimaryPart = game.ReplicatedStorage:FindFirstChild("RinkakuModel").Primary

local function makewelds(model,base)
local b = base or model.PrimaryPart
for i,v in pairs(model:GetChildren()) do
if #v:GetChildren() > 0 then
coroutine.resume(coroutine.create(makewelds),v,b)
end
if v:IsA("BasePart") and v ~= b then
local w = Instance.new("Weld",b)
w.Part0 = b
w.Part1 = v
w.C0 = v.CFrame - b.Position
end
end
end

makewelds(game.ReplicatedStorage:FindFirstChild("RinkakuModel"))

Local Script in StarterGui or StarterPack

local player = game:GetService("Players").LocalPlayer
local mouse = player:GetMouse()
local RinkakuModel = game.ReplicatedStorage:FindFirstChild("RinkakuModel")

local m
m = mouse.KeyDown:connect(function(key)
if key == "p" then
m:disconnect()
local clone = RinkakuModel:Clone()
local w = Instance.new("Weld",clone.PrimaryPart)
w.Part0 = clone.PrimaryPart
w.Part1 = player.Character:FindFirstChild("Torso")
clone.Parent = player.Character
end
script:Destroy()
end)