Spread glue on the two surfaces that will be joined together. Position the two parts of the miter joint on the flat part of the miter corner clamp so that their 45-degree angles are pressed together.
Tap the shims in between the piece you are gluing and the outside cleat using a hammer if you are using a miter corner clamp that uses shims rather than clamps. As you tap the shims in, the two parts of the miter joint will be driven tightly together.
Place a clamp on each half of the miter joint if you are using a miter corner clamp that requires clamps rather than shims. Tighten the clamps until you see glue squeezing out between the two pieces of the miter joint.