I don't know why the doctor would recommend this...unless she didn't want her getting attached to the crib which still makes no sense to me...but I'll still try to help! Like the first answer, be sure there's something there to soften her fall if she does roll off the bed. They make toddler beds that can turn into a "kid" bed when they grow up, they have the rails on the side and the most recent one I've seen gives A LOT of coverage so they won't roll off. But if you want to be safe, get a small pillow for her to sleep on, and move the pillow past the opening of the railing (that way she's completely surrounded). Give her blankets and her toys, she'll be fine. Of course, she will very quickly learn that she can get in and out and this bed! Just let her know that it's not for play time, it's bed time! And don't keep putting her back in bed...she'll think it's a game. She'll get used to it pretty quickly. Just be very positive about it..."Look! It's your new bed for night time!" That way she'll be excited to go to bed in it, not scared because it's something new.
When I would babysit this one little girl after daycare, she had just moved to a toddler bed, her mom had a regular pillow in her bed, the same blanket that was in her crib, and toys. She should be fine. (She was about 19/20 months when I saw they got a new bed for her).
Good luck! Hope this helps!