Little girl won't take timeout?!?

I babysit for a 5 year old little girl and her mom said to put her in timeout when she misbehaves. She gets really out of control when her mom leaves. She hits her brother, says inappropriate things, hits and kicks. When a ask her to sit in timeout, she won't go. How can I make her sit in timeout for 5 minutes and where should I put her in timeout (not in her room)?