How to make armor for a rpg or something

So I have tested welding a random part to a player and it worked but when I walk it moves back because its not perfectly with the position of the humanoidrootpart so now I am asking how do I weld a part exactly on the player

Use attachments to get the perfect relative position for positioning just like the default Humanoid accessory system.

You can use a normal weld or RigidConstraint, they function the same and the math is the same as well.

Rigid Constraint:

Weld formula which does the same effect:

well as said when using welds the part will move back a bit when moving