At one point I made REALLY fancy ones with two #2 pencils with big add-on erasers and paper clips bent into "L" shapes. I adjust the "L" in the eraser so I have the distance specified from the leading edge and add weights until it balances.

Normally masking tape on the desired balance points and fingertips. No matter WHAT the plans say I fly it to determine where the COG needs to be. The initial balance is just so it survives the maiden and first few trim flights.