Roll the basket on to the runner with the open end first. You want to bring it up t the back of the seat and stop.
Next, mark this spot on the outside edge of the runner (both sides) and drill through both the runner's outer edge and the basket runner inside. The hole should be able to easily accommodate the wire-lock pin, which will hold the basket in place while riding in this position.
Once the holes are drilled, slide in the wire-lock pins and give the basket a shake.
Hopefully you just said "that ain't goin' nowheres." Out loud. To nobody.