Vrooom Armament shows how to modify a Tokyo Marui VSR One chassis to add a Maple Leaf mag holder, which is not originally fully compatible. The process involves drilling a second hole in the mag holder for increased stability, ensuring it aligns with the M-LOK rail section on the chassis. The most crucial step is to dremel a cutout on the VSR One chassis to create a flat surface that allows the mag holder to sit properly and accommodate a magazine without interference. The video shows how to attach the mag holder using two M-LOK nuts and screws, ensuring a secure and stable fit. The modification allows for proper magazine clearance and provides a functional mag holder for the VSR One.