I do the ribcage straps first because they're the straps that are actually holding your costume on. Shoulder straps are mainly for adjustment purposes.
First I pin the ribbon onto the bra in the lower center. Then I wrap it around myself and safety pin it in the back (it's easier if you have someone to help you). Next I pin ribbon to the armpit area and connect it to the lower ribbon to create a triangle. After all ribbons are pinned in place, I undo the safety pin to take the bra off.
When that's done I cut two pieces of interfacing out that are the same size of the two triangles I just created.
Then sew it all together. This can be done on a machine. Hooray!
After you get that sewn, cover both triangles with your fabric.
Then just hand sew it to your bra. I use stronger thread for this because I don't want my straps falling apart while I dance.
If you take a look at the bra cups you'll notice that I also made 2 more beaded paisley appliques that match the one I made for the belt closure. I felt like the bra needed more bling.
Also not shown here; I added sequins along the border of my ribcage straps along with some more rhinestones from the Charming Charlie necklace I mentioned earlier.
The costume is almost complete!