Hello, 

I'm looking to make a variant race as a subrace like the aasimar. Can anyone tell me how that is done?

Try using an existing race as a template then add in celestial flavored abilities making sure not to make anything overly powerful.

Hi HyperViper, thanks for your question.

Is it a new race, subrace or variant you're making? You can create a copy of the appropriate one as a template when beginning to make your race. Please provide more details if you need any further help. You should also take a glance at this thread.