Applying fabric interfacing is a simple process that involves a few steps:

Choose the right interfacing for your project. There are different types of interfacing available, such as woven, non-woven, fusible, and sew-in. Choose the one that is appropriate for your fabric and the effect you want to achieve.

Cut the interfacing to the size and shape of the pattern piece that it will be attached to. Make sure to follow the pattern instructions or your own measurements carefully.

Place the interfacing with the adhesive (if using a fusible interfacing) facing down on the wrong side of the fabric. Make sure that the edges of the interfacing align with the edges of the fabric.

Use an iron on the appropriate temperature setting for the fabric and interfacing to press the interfacing onto the fabric. Hold the iron in place for a few seconds, and then move it to another section of the interfacing until the entire piece is fused. Use a pressing cloth if necessary to avoid any damage to the fabric.

Let the fused fabric cool completely before handling it. Once it's cooled, trim the edges of the fabric to the pattern shape.

If you are using a sew-in interfacing, skip step 4 and instead use pins to hold the interfacing in place on the fabric. Then sew along the edges of the interfacing, attaching it to the fabric.