Okay,. "RRR", this is how I was told to do it, and this is what works for me. IF you have a wireless modem, turn off the wireless part (I think it's called Bridge Mode), then connect an Ethernet cable from the modem to the router. Then using Airport software, see if you can extend your wireless network on your airport express. when you plug in the airport express, it should show up in that software, then clicking on it twice will open up the box that allows you to create or extend a wireless network. mind you, I tried to do this with Yosemite and it didn't work (Yosemite messed up my wifi), but on upgrading to El Capitan it worked. there's also a free app for your iPhone that works for this. Also if you click on your wifi network icon at the top of your screen of your mac
your airport express may show up and you can configure it that way. I tried to use a d-link back when I had Yosemite, but I got too frustrated, sold it, and bought an Airport express (3rd/4th generation, the 802.11n one) for $50 to go with my airport time capsule and bam !! wireless extended. I think the hardest part was using a paperclip and resetting the airport express, and waiting for the front light to go from yellow to green, but it did and it showed up, and it was all relatively painless. Also if you can, either change the channel on the Airport express, or set up a 5Ghz wireless network as 2.4 Ghz networks may be a little more prone to interference, but necessary....
good luck to you
John B