Write two verses in each bar.
A 16 bar rap structure in a song is made up of two verses in a musical bar as so [verse verse]. There will be 16 of these musical bars each containing two verses, which makes a total of 32 verses in one 16 bar format.
A verse is like a long phrase or sentence. In a bar one verse should ryhme with the next. If you can't find a way to make a verse rhyme with the other verse within the bar, then make it ryhme with a carry over verse.
To make sure you have a solid 16, you need to also create emphasis on certain words in each verse of a bar using one of the words in that verse. This technique is used, to get the most out of every verse.
Short Example:
VERSE VERSE
[And i stop at a light, pull off so slow] BAR
[But i'm out for the night, so pass that dro] BAR
note: verses from Ludacris's "Saturday" song
You also want to make sure, that there is the same amount of beats within each bar, If you decide two beats per bar, then you need to stick to that. It makes the song flow much better when everything is organized. That is the whole point of 16 bars, to make rap songs more structured, catchy, and smooth.