When you’re ready, share the job description with everyone you know locally.
After that, post it to social media (don’t forget LinkedIn!). If you are looking for a studio assistant, artist forums might be the best place to focus your efforts.
Don’t sit back and wait for applications to roll in. You have to keep talking to people. The more people there are who know about your search, the more likely you are to find the perfect person.
I’ve never had to do more than this to get excellent candidates, but I imagine LinkedIn might be a great place to find qualified administrative assistants. Artist forums could be useful if you’re seeking studio assistants.