I agree there should be some signs for that because for a complete noob to prison like me I cant figure it out
when there times is up i think you go up to the cell right click the sign and you get it it's like 7 days for 10k
There is notifications in the chat but when a cell is open you can right click the sign and buy it for 10K and you get to keep it for 7 days. Just keep this little tip in mind, from experience, there will be more than one person going for that cell but if you are lucky, you just might get it.
I agree that there should be signs explaining how to get cells and even plots, when i was a noob in prison it took me quite awhile to understand how it worked. Also many people still do not understand how it works.
When the cell is for sale it will say "10k for 7days" on it, so that means its 10k for every 7 days you rent that cell