I continuously sent out our clothes to the laundry onboard the Magic. When we first got on the ship, I sent out all of DSs' and DH's dress pants, suits, dress shirts, and my dresses to be pressed. They were returned the next afternoon.
To answer your questions:
1. There are plastic bags in the cabin, as well as price lists that you need to fill out and check off how many of each item of clothing that you are sending, as well as any special instructions. On the form you can also say if you want the items pressed, washed, or dry cleaned. I asked our cabin steward for more plastic laundry bags because there were 4 of us. The bags are not very large to begin with. Our cabin steward told us to just leave the filled laundry bags on the bed and that he would take them each evening. So that's what we did. One time I wanted to send out clothes after he had already done the evening turn-down service, so I called him on the phone (there's a button to push to reach your steward), and he came and picked the bags up. Our cabin steward was Darwin, he did an excellent job, and we didn't hesitate to leave him an extra tip at the end of the cruise.
2. Sometimes the clothes were brought back while we were in the cabin, so I'd tip the delivery person. Other times the clothes came back while we were out, so I wasn't able to tip. Some of the clothes will be returned, washed, pressed, and folded, in a laundry bag, other items will come back on hangers they will place these right in the closet, if you're not there to accept them.
3. For regular service, the clothes will be back to you by the following afternoon. I believe it says "Before 5pm". There is same-day service, too, for an additional charge.