Ikea Varde wall cabinets are 27.5 inches wide. Most wall studs are 16 inches apart. Some, like our one wall are 24 inches apart, which is actually just right for the Varde wall cabinet hardware. We read several suggestions, including cutting out the wallboard, mounting a 2X4 onto the studs, replacing and patching the wallboard then continuing from there. That was a no go for us, since any number of things could go wrong, and end up in expensive repair. We decided on a different route, using plywood instead.
Step 1: Plywood Base
We went to Home Depot and bought regular 3/4 inch plywood. You can get birch plywood, and if I had the option again, I would spend the extra money. We had Home Depot cut the board to the height we needed, and we cut the lengths ourselves, allowing an inch or so if needed.
We marked the studs on the wall. One wall had them 16 inches apart, and the adjoining wall had them at 24 inches apart. We measured from the corner, and marked our plywood accordingly, adding in markings for the Varde hardware supports. We did this by laying the plywood down on a flat surface (garage floor) with cabinets in position, aligning the tops of the cabinets flush to the topside of the plywood. Then we traced the Varde bracket positions onto the plywood using black Sharpie.
We then mounted the plywood onto the studs, and placed the IKEA bracket screws in their place, as well.