Measure the length of the straps you are replacing or the length you require for a totally fresh strap. Say the length required is 18 inches. Draw a 1-by-18 inch-rectangle onto microfiber fabric with standard chalk.
Cut out the strap along the trace lines with a pair of fabric scissors. Draw and cut a second 1-by-18 inch rectangle onto microfiber fabric strap as necessary.
Fold the strap in half to 1/2 by 18 inch dimensions on an ironing board. Iron both sides of the strap to reinforce the fold.
Stitch the long side of the strap together using a back stitch pattern. Use thread that matches the color of the strap.
Fold the left side short end 1/4 inch over the body of the strap. Sew the 1/4-inch piece to the strap using a cross stitch pattern. Sew the right side of the strap into the bikini or other clothing using a cross stitch pattern.