Well, first order of business, if you're serious, would to stop the fentanyl.
Pretty much once you're in full blown withdrawal, you can take the suboxone. A rule of thumb I always used once I hit at least the 18 hour mark since my last shot, I'd wait until I absolutely couldn't bare it anymore, wait one more hour and take a sub.
With bupe, less is more. For detox, 8mg is just as good as a jumpoff point as 16mg. I wouldn't take it more than 5-7 days either. Longer than that you run the risk of getting sick from bupe withdrawal which, chances are, you'll go straight back to the dope. Or continue using bupe long term, which is debatably worse in terms of withdrawal.
A regimen that always tended to work for me, personally, which is also pretty standard in most detox facilities is as follows:
Day One: 8mg
Day Two: 4mg in the morning, 4mg at night
Day Three: 4mg in the morning, 2mg at night
Day Four: 2mg, AM/2mg,PM
Day Five: 2mg, AM
If any of the following are available, id definitely supplement these as well:
Valium, Librium, low dose Klonopin for sleep. Maybe some Gabapentin.
Marijuana can help with cravings, appetite, and over all mood, but I know that's not everyone's thing.
Immodium as needed for diarrhea. Though, personally, I get extremely contributed when I take bupe, so you may need a stool softener instead.
Even after 5 days, of bupe you're still not going to be 100%. The biggest issue at that point is insomnia, so definitely implement whatever can help you sleep at night. Rest makes a huge difference. Withdrawal peaks around 3-4 days so remember, as shitty as it may be, you're already in the home stretch. If you use at this point you will be as sick as you were on day one. Just chill out, watch some mindless TV, eat some healthy foods, take long baths, do some cardio if you feel up to it. Keep positive thoughts, as long as you're still alive, you're not doomed to that life. You can do it as long as you're determined