3 solutions, top = best
1) run ethernet cable from primary router to second router in basement.
2) use powerline adapter from primary router to second router in basement.
3) get a wireless bridge to recieve wifi signal, and then plug it into secondary rotuer.
As far as configuration it depends on your nework access/security goals.
If you want to allow them to access shared files/printers then you will want to configure the second router as an access point.
If you do not want them to have access to shared network files/printers then you will have to do some configuring. Your options vary depending on what options your router has, and if your router can be upgraded to dd-wrt or not.
Some routers allow you to set access restrictions, and some higher end ones or ones with firmware like dd-wrt allow you to setup a VLAN (virtual lan that provides sperate issolated networks), and that will issolate all traffic of your renters router from your own.
The other option is to have a third router. One router for the internet, one for your devices, and one for the renters. This will keep everyones network issolated without any reflashing of routers or heavier configuration of things like VLANS