Originally Posted by CaptainSTX
1. Turn off DHCP on router you want to make into an AP.
Before that, give it a static IP address so you can still reach it for configuration.
Can be done by setting an explicit static IP or setting your real DHCP server to give it a static IP based on MAC (I'm using both ways because one of the routers I use doesn't support DHCP client on LAN when set up as access point).
But I agree that buying a router to use as access point is usually a cheaper solution than buying a dedicated access point.