The fabric needs to be the same width as your desired bag, plus 1 inch (2.5 cm) for the side seam allowances. You should also add 1 inch (2.5 cm) to the overall height for the hems.  For example, if you want a 6 by 12 in (15 by 30 cm) bag, your fabric should be 7 by 25 inches (18 by 64 cm). Use a sturdy fabric, such as canvas, cotton, linen or canvas. The strip can be whatever length you want it to be, but it needs to be twice the width, plus a 1 in (2.5 cm) seam allowance. You can cut 1 long strip to make a shoulder strap, or 2 short strips to make handles.  The strap or handle does not have to match your bag. You can use a contrasting color to make your bag look more interesting. Use sturdy, woven fabric for this, like cotton, linen, or canvas. Don't use stretchy fabric. Cut a 1 by 1 in (2.5 by 2.5 cm) piece of Velcro. Find the center of the front and back top hem. Glue each piece of Velcro to the inside of your bag, right against the top edge of the hem. Wait until the glue dries, then press the Velcro together to close the bag.  Avoid using self-adhesive Velcro. The glue will eventually come off. For best results, use fabric glue. You can use hot glue in a pinch, however.

How would you rephrase that briefly using Indonesian?
Potong selembar kain dengan panjang dua kali ukuran tinggi tas diinginkan. Potong selembar kain panjang untuk pegangan atau tali bahu. Tambahkan Velcro sebagai penutup jika Anda menginginkan tas yang bisa dibuka dan ditutup. Selesai!