The pictures that Ralph and Thomas showed are pretty typical. But in the end, you need to look at the specifics of your situation to decide the best wall to do a wall mount.
In my former darkroom, I used a mounting arrangement similar to what Ralph and Thomas showed - a narrow shelf supported on brackets (korbels?). This made it possible to then have a shelf in front of the mounting shelf that held the enlarging easel, and that could be shifted lower in order to make bigger enlargements. I needed that because I had a low ceiling in my basement, and didn't have a lot of head room to elevate the enlarger head.
In my current darkroom, I have plenty of headroom, so I built one shelf to hold both the enlarger and the easel. However, in this case the enlarger is in a corner, so the shelf is supported on cleats attached to the two walls that come together in the corner. I have cabinets adjacent to the enlarging station, and that are support cleats on them also, resulting in a pentagonal-shape shelf supported on four of its five side (the fifth side is open and is the 'working face').
Two cautions. First, you need to make absolutely sure that the support system (brackets or cleats) is attached firmly to the studs in the wall behind the enlarger. I used lag screws to attach my supports - I didn't want to trust simple sheet-rock screws or nails.
In my first experience, where I had a shelf that was attached to only two studs, I added a cross member between those studs (similar to a fire-stop) behind the sheet rock to assure that they could not twist relative to each other. That cross member provided additional rigidity to the installation.
Second, for a large enlarger that is wall mounted, you have the opportunity to add a bracket near the top of the enlarger column. In my experience with Omegas, big enlargers tend to wobble a bit. You can learn to live with some wobble, but it makes life much easier to attach a brace between the top of the enlarger column and the wall that makes the system totally stable.
