3 point strap, how do you use this?

So I purchased a standard 3 point elastic strap.
What I am confused on or I think might be an issue is I can't get the bottom part on.
I think my thighs might be a little too thick? I also don't know which way im meant to clip the button around.
Can someone please advise? or if you know of an alternative that might work with my larger thighs if thats the issue?
The normal belt part is fine but Id like try the 3 point one