I recently finished setting up my Pi with Raspbian Stretch lite and configured the Wi-Fi successfully. I was trying to assign a static IP address to it, but all I can find are tutorials for LAN not over Wi-Fi connections. I was hoping to take the easy path and assign the IP through my router, but it doesn't have that option.
It is basically the same way you would do it for your ethernet interface, you simply need to use the name of your wireless interface (presumably)
In Raspbian stretch (lite), if you add this part to your
/etc/dhcpcd.conf you should be fine:
interface wlan0 static ip_address=192.168.1.99/24 static routers=192.168.1.1 static domain_name_servers=192.168.1.1
Of course you need to adapt the IP-addresses to your specific setup.