They are a long cook and because I start with a 250° pit temp, mine take at least 8 hours. Prep is minimal, trim off any tags that might burn and no need to remove the membrane. Really no reason to season the bone side, but I will add some cracked pepper. Season the meat with what ever suits you, but at least salt and pepper. If you want to use a mustard slather, try thinning it out with pickle juice. I smoke mine bone down the entire time, even for the wrap step. After 2 hours I'll spray them a couple of times before wrapping. For reference, this rack is on a 1/4 sheet pan, so the bones are about 10" long.
After 4 or 5 hours I'll get the color I want, and the bark is set... the internal might be in the 160°'s. And I'm hardly getting any pull-back.
Here is a close-up right before wrapping
The wrapped step will take 4 or 5 hours, and you want to
pay attention to both tenderness and pull-back. Open the pouch after 2 or 3 hours to get an idea how they are doing. Many people get anxious and pull them too early, but be mindful that they take a long time to tender up.
Don't use internal temp as a guide for doneness, you want to cook them tender by feel (probing) , just like you would a brisket or pork butt. Think of them as a brisket point an a stick. Some people take them to an almost shredable state, I don't go quite that far but I've had the bones fall off when slicing.
I rest mine at least 2 hours in a hot box, longer if I can get away with it. If you slice too early you will loose valuable juice.