Halp! Trying to install a seat bag with no instructions

[zoidberg]

This bag - www.performancebike.com/reviews/nashbar/power/pwr/product-reviews/Brands/Louis-Garneau/Louis-Garneau/p/LG__MSTRM__BASE-Louis-Garneau-Mini-Stream-Seat-Bag.html

I have it clipped into the rails under the seat, but for the life of me I can't figure out what I am supposed to do with the elastic band.

[katinthehat]

it loops around the seat post if you take your saddle off

[zoidberg]

Ooooooh, ok. Thanks!

[katinthehat]

probably not necessary but it's an additional layer of insurance

[zoidberg]

I think I got it, it just looked like it should go a certain way but I don't think that was right. Why does none of this stuff ever come with instructions?