I've been raising bum lambs since 1994. The first thing you have to learn is that lambs are born to die! NO JOKE. Everything eats them and their give up factor is around 90%.
Whatever you choose for a replacer, I prefer goat's milk, keep the mix consistent by weighing the powder and using the exact same measuring cup each time for the water. Decide whether you are feeding hot, warm, or cold water and don't change it. The biggest loss in bums is scours and bloat. Those are usually caused by changes in diet, but they can also be caused by stress. Also, your feeding times for the first week should be fairly exact. After a week it is okay to vary by 30 minutes or so, but not much over that. If you miss a feeding, DON'T give extra.
Lambs count on others to stay warm. They snuggle with their mother or pile in groups. I use dog coats from Wal-Mart that I got for a buck to keep them warm even if they are inside. These coats work well on the girls; however the boys are a bit harder. If you wait to move them outside until later, you will have over a week of no sleep at night and no peace when they can't see you during the day. Start them where you want to finish them, just make it smaller.
Love on your lambs, but don't let them climb on you. I just put a very nice ewe in the freezer because she learned to jump on me as a bum and kept on as a 125 pound adult. As I also learned early, "bum rushed" is a very real problem. 10 or more bums clambering at once to be fed can create quiet a tripping hazard.
Lambs can be enjoyable. They are a lot of hard work and make very little profit. Good Luck.