Step 1: Right sides together, sew your seam as you normally would, with a 5/8" seam allowance (or what the pattern calls for). Press the seam open and sew a strip of single fold bias tape to one of the seam allowance, matching raw edges and right sides together. Be careful not to sew through the rest of garment.
Step 2: Flip the seam allowance over. Fold and press the bias tape toward this side and over the raw edge, making sure to cover the stitching line made when attaching the other side of the bias tape. It may be necessary to trim the seam allowance slightly if the bias tape does not reach. The bias tape should remain folded with its raw edge tucked underneath.
Step 3: Flip the seam allowance back over. Stitch "in the ditch" (where the bias tape is stitched to the seam allowance), making sure to catch the other side of the bias tape in your stitches.
Repeat for other side of seam allowance.
Variation: Instead of stitching in the ditch on step 4, sew a decorative or zig-zag stitch. This is easier than stitching in the ditch, and is still fun and decorative.