Best way would be to operate the Sigma in manual mode, dialing in the strength you need via the menu. Ensure the menu does NOT display AA or ETTL etc (The menu system on these are renowned for being a faff).
Mount in your softbox of choice,
possibly something along the lines of this one.
Now, you have two options to synch this flash:
1) hard wired (easiest to do)
Buy a long pc synch and conect the two, job done

What? No synch socket on your Sigma?
Then buy one of these from FleaBay
2) slave triggered
I'm not entirely sure, but I think the 530 Super has a slave option, meaning it can be set off by another flash. Which means you can trigger it from your pop-up (I'd dial down the power of your popup so as not to influence the pic too much)
If it doesn't have a built in slave mode, then use the widget from FleaBay indicated above, as that does have a slave
3) radio triggered
Cheap ebay set of triggers. One sender and one receiver, Jobs a goodun
The Sigma Super range are actually very well specced as off camera strobes, and have full manual override. Unfortunately, neither the manual, nor the menu are very intuitive.
This may help you retain some sanity and hair!
Let us know how you get on