Measure the dimensions of the opening you want to cut, and use them to outline where you want the opening on the non-load-bearing wall.
Locate the studs that you'll cut for the opening. If the wall is covered with drywall, then use a stud finder according to its manufacturer's instructions to locate the studs. Use a hammer to knock out drywall within the outline you marked on the wall, and look for pipes and wires that will be affected by cutting the opening. Consult a licensed plumber and electrician if you are uncomfortable cutting pipes and wires or to follow building codes. The pipes and wires should be dealt with before cutting the opening.
Cut with a circular saw along the outline you drew on the wall. Remove the cut studs and insulation in the stud cavities.