The 1st and 2nd sentences in the requirements is the beginning of the program. This is known as a flip-flop circuit or control. It is much simpler to program two buttons, one for starting and the other for stopping. Many HMI's (Human Machine Interface) have the option to be momentary or Toggle On-Toggle Off.
Another thing is I think the Entrance conveyor and Exit conveyor control should have been included within the scope of this project.
To help you understand what is actually going on, (generally speaking) treat each sentence as a rung or section of the program.
There is nothing wrong to state that this is a homework problem . Most of the forum members will recognize it as such anyway.
It is better to show whatever efforts (even they are incorrect) to demonstrate what level of understanding that you have.
Another thing, it is good to state what plc brand and model you will be using as there are many.
Again, please show us what you have attempted so far.
Edit:
It appears James was posting at the same time. I agree with him 100%.