There is many ways to do it, a fast and maybe crude way i do it is bolt the blades togheter with a fairly long bolt, then i spinn it up in an old gws eninge shaft that has ballbearings and add pieces of tape untill it balances exact. You can do this by hanging it in a piece of string also. More precise ways include special balancing tools or making out the CG point on each blade then compare and add tapes outside the cg point of the blade,, this can be veeeery timeconsuming.