It's a much easier trip from Belize City to Flores than vice-versa. Guatemalan tour mini buses leave the Belize Marine Terminal every morning at 10:00am. The ticket price is $20.00US and the trip takes less than 4 hours and is quite pleasant.
Unfortunately, the return journey on the same mini bus means you have to get up early...the mini buses leave Flores at 5:00am.
If there are 2 or more of you you can get a taxi van from Flores to the Belize border for well under $100.00US and then just grab one of the many frequent buses from the border to Belize City.
I do NOT recommend taking the "chicken bus" from Flores to the Belize border and unfortunately to the best of my knowledge there is no "Pullman" (Guatemalan first class bus) between those destinations.
The Belizean "Chicken buses" are much nicer and less crowded.