The first step in making a sentence flow is to divide up the passage into the individual units of thought, called propositions.
A special case is compound sentences. A compound sentence either has more than one subject doing the same action (verb) or the subject doing more than one action (verb). If it is more than one subject, doing the same verbal action, treat it as one proposition. If it is the same subject doing more than one action, treat it as separate propositions. The number of verbal actions determines the number of propositions.
Jim and John threw the ball.
This sentence contains only one proposition because there is only one action.
John hit the ball and ran to base.
This sentence contains two propositions:
John hit the ball.
[John] ran to base.
The number of verbal ideas determines the number of propositions.