How do I weld big parts to a character's hand?

I've tried using weld constraints, welds, and attachments but they always end up glitching the char out. I'm not sure what else to do at this point. Here's what keeps happening: https://gyazo.com/bfbd104efdef01dcbe29fe90c3f12d79

Here's part of my script as well:

local s = game.ReplicatedStorage.RedBlade:Clone()
	s.Parent = game.Workspace
	s.CFrame = char.HumanoidRootPart.CFrame
	s.CanCollide = false
	s.Anchored = false
	local w = Instance.new("Weld")
	w.Parent = char
	w.Part0 = char.RightHand
	w.Part1 = s
	w.C0 = char.RightHand.CFrame
	en = true