This link should help:
Tie the bottom of legs or plant in the garden where they are to remain using something to hold them upright whilst you fill them with gravel/stone/rocks and then soil. You could also fill them with concrete mix up the legs and place a container inside the body to take the soil and plants. Have fun...
Here's a video tutorial: https://www.youtube.com/watch?v=ioZxip55fXs
There is a site called Pillar Box Blue that has ideas for anything you might want to make with old jeans. https://www.pillarboxblue.com/