i recommend you position it how you want it in studio and then use the command bar to find its offset to the torso
[in command bar]
print(workspace.DummyTest.Torso.CFrame:Inverse() * workspace.Katana.CFrame)
then in the script
Katana.CFrame = Torso.CFrame * CFrame.new(whatever the command bar said)
local weld = Instance.new("Weld")
weld.Part0 = Katana
weld.C0 = katana.CFrame:Inverse()
weld.Part1 = Torso
weld.C1 = Torso.CFrame:inverse()