This is a really easy and fun DIY that can be done with a sewing machine or hand-sewn. The idea is to take a plain cami (like these below, which are $8 at Target) and make it special. You can use all kinds of tanks and ribbons to create completely different looks.
Pick out some ribbon that you fancy. I picked the rainbow ribbon to go with the navy cami and I'm planning on using the ivory lace on the white cami. You might also consider buttons, bows, rosettes, or other tidbits.
Fold about an inch of ribbon under so it won't fray and use your sewing machine to secure it to the back of the cami, lined up with the strap.
From here on out, I stitched everything by hand. After I tried on the tank and made sure the straps were adjusted the way I wanted them, I secured the ribbon to the strap every couple of inches with a stitch or two. It looked like this when I finished securing the ribbon to the straps.
I continued to use single stitches to secure the ribbon along the neckline until the ribbon ran out, at which point I tucked it under itself and sewed it down. Then I cut a piece of ribbon to fit the remaining space, folded the ends (again, so they won't fray) and stitched it down to meet either side. You can see where I used navy thread below. (Red would have probably been better to blend in with the ribbon, but I don't really care!)
And you're done. Mine isn't perfect. I wish I'd thought ahead so that the ribbon matches up on the left side but it's barely noticeable. Unless someone, like me, points it out. I'll let you know how the white cami turns out too.