mobile

Hey all, so I bought a nexus 5 about a week ago(loving it!) and it came with boot loader unlocked. Basically I'm tryna install twrp through fastboot so I can flash SuperSU and root. Andriod version is 5.1 lollipop and I'm trying to install 2.8.5.2 twrp. I can get cmd to recognize the andriod SDK platform and is recognizes the fastboot recovery command but it says waiting for device. Do I press start on my phone or should it do this itself?