I lay a blanket or small quilt down on the floor in a location that is quiet (and safe) for her and is not right around me. I let her choose one toy (or set of related toys like her dollhouse or Mrs. Potato Head) and one book and take them to the blanket. Next, I ask her the rules of blanket time, to which she promptly (and happily) replies, "Rule number 1 is 'Sit on my bottom.' Rule number 1 the second time is 'Be quiet.'" Then I set the timer for 20 minutes (we'll probably start doing 30 minutes soon) and let her push the start button. That's it. I walk away, she plays, and there is peace and quiet for 20 precious, consecutive minutes!
A really nice benefit of Ava liking Blanket Time is that I was able to implement it in public when Tim and I had an important professional meeting and we were unable to do anything but take the kids with us. On the way, I explained to Ava what she could expect when we arrived at the office, and when we got there, she was perfectly content to sit on her blanket (I took the smallest one we have) beside our chairs and play quietly for the duration of our meeting. I was so proud of her, and I was thrilled (and relieved) that it worked!
One thing I am careful to do is to not use Blanket Time as a punishment. I only treat it as a positive thing. So far, so good! She is often the one reminding me that "It's Blanket Time!"