Step 1: Materials
You will need a tank top with wide straps like shown. You will also need about a fat quarter of fabric. Cotton, preferably. Last you will need fusible interfacing and thread in the color of your choice.
First start by ironing on the fusible interfacing to the back of the fat quarter of your fabric.
Then fold the fabric in half with the interfacing facing out
Print out this pattern if you don't want to freehand it and trace or pin this pattern on your fabric, interfacing side. Line the folds up.
After you've cut it out, draw your own design on the rest of the cutout.
To cut out you can use an exacto knife but the faster method is the old-school snowflake style. You fold each of your designs in half and cut them out.
It should look something like this when you are done.
Now for the long and kind of tedious part: Sew around each of the cutouts. Also sew around the entire thing with a zigzag stitch to prevent massive fraying. It will still fray a little though, expect that.
To attach it to the tank, cut off the part of the strap you want to replace.
The fold over any new raw edges, pin, and hem.
Place your cutout in the spot you want it, overlapping the strap a bit for a secure attachment. Pin in place and sew.
It should look like this now.
And here's the finished product!