One of the criteria for getting an OPT extension is that your employer should be enrolled in E-verification program (besides the obvious post-completion OPT status and STEM degree). Make sure that your employer participates in that (if it's not then you could request them to participate). The rest of the process is, I think, pretty similar to how you applied for your OPT. You will need to contact your DSO regarding that and he/she can tell you how to go about it. You should file for an extension before your OPT expires.
As for me, I am eligible in every other aspect besides my employer's participation in e-verification program. It's just too bureaucratic of a company and it takes more time than I have for them to make a decision about participating in the program. This clearly sucks and there's nothing more I can do besides go back to school or go back to Nepal. Well hopefully things will work out for you. Good Luck!