Check out this outstanding Brownells video on how to attach MLOK accessories to MLOK slots. While they are installing a Picatinny rail to their handguard, the process is the same for ARCA.
35 INCH LBS FOR ATTACHING AN ANNEX ARCA RAIL TO A METAL HANDGUARD. 15 INCH LBS for attaching to a plastic handguard.
For those of you mounting to a gas gun (AR-15 / or similar) with a gas block, take note of making sure the screws are not contacting the barrel or gas block: