Congrats on the Bci!!!!
However, no pictures Evil man!!
So, you'll want to make sure the temps are a bit lower than what you'd keep for a ball python but the humidity is pretty much the same.
As for feeding (and as was mentioned) once a week is fine. Bci's don't grow as fast as ball pythons and they are not circular in body shape like balls are. I have two yearlings (600g+) and they are taking f/t weanling rats. I have a 4 month old who is taking f/t rat pups.
I think you'll be able to tell if the snake is still hungry!! Bci's have terrific feeding responses!!! Since you believe yours to be underweight, start with a smaller sized prey for a couple of weeks and work up to what you think it ought to be having.
Since it might not have had the greatest of care with its previous owner, I would recommend a vet check with a fecal float (when the snake gives you one!)
I have never left my boas alone after getting them. They are curious creatures and I like to let them get accustomed to me as part of their environment; a long-time breeder suggested this to me with her boa and it worked great.
Of course we are all expecting pictures as soon as you can get them!!!
Congrats again. Boas are great snakes and you'll have a lot of fun with yours!