Writing bitfields with varying field lengths

[Grumfish]

In order to familiarize my self with Flash files and their bytecode I've 
started to make an assembler. My first problem is writing the bitfields 
the format uses often. It is a series of fields, each can be a different 
number of bits, combined into the least amount of bytes possible. Extra 
bits in the last byte are padded with zeros. I would like to make a 
function that takes a size and value for each field needed, calculate 
the amount of needed bytes, place the values, and the nretun a binary 
string of the resulting bitfield. I am at a complete loss on how to 
place the values into the field. I've Googled but have found nothing 
helpful. Can anybody help?