There is no hard and fast rule that says you have to use wall cabinets for the upper cabinets and the base cabinets for the lower cabinets. It is just a common configuration for standard kitchens. However, there are lots of uses for wall cabinets to be used as base cabinets. Think about using them for window seats under a sunny window wall. Maybe try lining a thin hallway with thin wall cabinets. If you don't need counter space just fill a whole wall with wall cabinets, or use a thin countertop instead of the 32-inch deep cabinet.
Set your cabinets on the floor where you want to install them. Make a scant outline on the wall and along the floor with a pencil. Remove the cabinet and then find the studs in the wall. You can use a stud finder if you have one, or just a long thin nail that you poke every few inches until you locate the studs. Mark the studs with the pencil.
Lay a 2x4 flat on the floor the same width of your cabinet along the front edge, but set back about 1/4 of an inch. Set one flat on the floor flush with the wall where the back of the cabinet will sit. This will lift the cabinet off the floor a little. Screw both of them in place with the wood screws. Sink the screws into the wood so they are flush and not sticking up.
Set the cabinet on the 2x4 base. Mark the inside of the cabinet where the studs are you found earlier. Drill a pilot hole through the cabinet and then screw a wood screw into the hole. Place two screws on the top edge and two on the bottom edge. Your cabinet is now secured with the wall.
Drill two more pilot holes through the cabinet and into the front 2x4 on the floor. Screw in two more wood screws. This will hold the cabinet securely in place.
Measure the area at the bottom of the cabinet and cut a piece of cabinet veneer to cover the 2x4 to match the cabinet. Glue it in place and secure with finish nails.