In order to setup nameservers you need to do two things:
1. create the nameservers with the registrar
2. create A records for the nameservers
In order to do #1 with OpenSRS, login to your http://manage.opensrs.net page and go to the Nameservers page. Click the link at the very bottom about creating nameservers. Add the nameservers based on your domain and point them to the correct IPs using the form at the bottom of the page.
In order to do #2, you should create A records for your nameservers in the spot where you'll be managing DNS when you change your nameservers *and* in the spot you're currently managing nameservers. Add the A records for the ns1 and ns2 records pointing to the same IPs you pointed the nameservers when you created them at the registrar. Then I'd recommend waiting around 12 hours or so and then update the nameservers for the domain to your custom nameservers.