Yes, just plug old router that you want as access point into a spare Ethernet port on main router (should not matter which one).
You are correct in turning DHCP off on old router. That is a must.
You do not need to turn off the broadband section as no cables will be connected to this and client PCs will not be attempting to use it to access net (because will be setting gateway on client PCs correctly as described below).
Next thing make sure it has a sensible IP address that does not conflict with anything else on network. Need to give it static IP but will obviously have to be different to main router that does your broadband connection.
e.g. main router could be 192.168.0.1, access point router could be 192.168.0.2, make sure neither of these within DHCP pool on main router.
If you have any PCs on network with static IP addresses need to set gateway to main router IP address not access point IP address. Set by clicking on adapter settings then TCP/IP properties. Subnet mask should be 255.255.255.0. Any PCs that use dynamic IP addresses should just work as main router would set itself as the gateway when it answers DHCP request when you initially join the network.
