AFAIK /etc/network/interfaces is mainly a Debian and descendants thing. I don't have that folder on any of my arch machines.
I'm assuming you're not talking about WiFi here. If you are, have a look at netctl or systemd-networkd.
Assuming you want static configuration, create /etc/systemd/network/50-wired.network with the following content (changing Name, Address and Gatway according to your network, of course):
[Match]
Name=enp1s0
[Network]
Address=10.1.10.9/24
Gateway=10.1.10.1
Disable NetworkManager:
# systemctl stop NetworkManager.service
# systemctl disable NetworkManager.service
Enable and start systemd-networkd.service:
# systemctl start systemd-networkd.service
# systemctl enable systemd-networkd.service
After you have configured dnsmasq to do what you wanted it to, you activate and start it:
# systemctl start dnsmasq.service
# systemctl enable dnsmasq.service