I used leg mounting brackets. You can see them lying on top of the shelf in the before pic (they are basically metal squares with 5 holes. The 4 corner holes secure the plate to the shelf, and the 5th center hole is to screw the legs into. So you will need to drill small guide holes for the 4 corners and one bigger hole to screw in the leg into the center hole). The problem though it the kallax is actually made of hexagonal cardboard on the inside with a small wood layer on the outside, so none of the screws held very well. I ended up buying some industrial strength epoxy at Home Depot and putting it into all the corner holes and attached the plate. Then I poured the epoxy into the large center hole and all over the plate itself before screwing the legs in. It feels super sturdy now but the only downside is the legs can not be removed. I bought the legs on Etsy. Just make sure the bolt that comes attached to the legs matches the size of the center hole on the mounting plates. The standard in the US appears to be a 5/16 bolt.