Self Install means no engineer enters your premises, its all done externally.
For fibre its highly recommended you connect the modem directly to your master socket.
If your BT Master box already has an RJ11/45 socket built into it you're pretty much good to go.
If however its your typical NTE5 master with just a standard telephone socket, whilst you could just use the modem supplied dsl/phone splitter accessory that plugs into that phone socket, you are far better off buying an Openreach MK3 faceplate to attach to your master socket, as this will give far better results.
I noticed prior to using a MK3 faceplate I was getting around 50-250 packet errors a day, which actually isn't bad, but once I installed the faceplate and connected directly to its RJ11/45 port, my errors went down to 0, and are still at 0 after 10 days and counting.
Your download speed will mostly likely improve as well. You can order the Openreach MK3 faceplate off Amazon. It's dead simple to install, no need to mess with any wiring you just unscrew the existing faceplate part of the NTE5 box off, slot the MK3 faceplate in between and screw back in over the top