well, you don't "attach" it, really. get a length of line (though 1" nylon web strapping won't dig into your shoulder so badly) about a foot longer than your arms, and secure one end to the straps of the bag. rig a carabiner onto the free end. this allows it to be stowed until the inevitable capsize. after you crash, simply stand on the lower hull, snap the carabiner onto the righting line, squat down and fill the bag with water, and then get your shoulder under the line and stand up/hike out. the carabiner should slide up the righting line with your hands.
now, this is a very simple rig, and it only works because you specified that you bought a "small" bag. larger bags are too heavy to simply stand under, and require a bit of block and tackle to properly rig.
