How do I setup Netgear R6250 main, and the Netgear DM111PSPv2 as signal booster?

I have a Netgear R6250 and a Netgear DM111PSPv2. The R6250 is in the lower level of my house, but the signal is not very strong in the opposite end, upper level of my house. I was given the DM111PSPv2, and I thought I put it to use to somehow boost/repeat the signal. I've tried to read about how to do that, and I seem to get lost each time. I have only Macintosh computers running Yosemite 10.10.5. I do have an Ethernet cable that is already run from the R6250 to where I want to place the DM111PSPv2. I bought one of those Belkin, range extenders that plugs in to the wall, but it seems to cut in and out all the time. I assume I will retire it if I can get the 2 Netgear routers to work. All the instructions I find seem to be for PC and/or different modems/routers, and inevitibly I always get lost and frustrated. I don't know all the terminology, so please keep it simple.   One other thing, how do I get routerlogin.net to choose one versus the other router? 

Don't know if this is important or not, but...

I have 2 computers (rarely use, so slower connections would be okay) & 2 laser printers, all downstairs, that need to connect via Ethernet. I have 1 MacBookPro and 2 iPads that need to connect via WIFI (usually upstairs). IF it helps/matters I have an old Asante 10T Hub.