Abby's attorneys tell us she checked in around noon at the federal facility in Victorville, CA. The former "Dance Moms" star is now on day 1 of her 366 day sentence. A Lifetime camera crew -- reportedly shooting a news special on her sentence -- followed her as she went behind bars.

We're told she was pretty nervous as she arrived for processing. She could be out within 10 months.

Unclear how long it will take her to get squared away in her cell ... but we know this: meal time is gonna be a culture shock for Abby.