OK, well it's YaST2 > Network Services > Network Card, then click the Change button.
Select the device you have static IP for from the list, and click Edit.
Change from Automatic Address Setup to Static Address Setup, and give it your static IP and network mask (mybe someone else can explain network masks - I don't fully understand them myself)
Click Next > Finish, and you're done.
Since I've done this myself, my internet browsing has slowed down significantly. I think this is because I messed with the nameserver settings at the same time, so it takes longer for URLs to resolve. If that is the case, then I THINK that my instructions won't affect your performance. Just be warned.