How do I use /etc/network/interfaces instead of network-manager

If you manually manage your network card in /etc/network/interfaces , network manger will not manage it , it will state "Not Managed"

Suppose your network card is eth0 :

To setup eth0 to static, enter:

open /etc/network/interfaces :

auto eth0
iface eth0 inet static
address 192.168.1.15 #------> Your Ip Address
netmask 255.255.255.0 #------> Netmask
gateway 192.168.1.254 #-------> Gateway
broadcast 192.168.0.255 
dns-nameservers 192.168.1.3 #-----> Dns server

To setup eth0 to dhcp, enter:

auto eth0
iface eth0 inet dhcp

The different keywords have the following meaning:

auto: the interface should be configured during boot time.

iface : interface

inet: interface uses TCP/IP networking.

Now restart service :

sudo service network-manager restart

In the file /etc/NetworkManager/NetworkManager.conf:

[main]
plugins=ifupdown,keyfile
dns=dnsmasq

no-auto-default=00:0C:29:90:24:0F,00:0C:29:2E:C8:2C,

[ifupdown]
managed=false

where false means that network-manager doesn't manage the interfaces located in the file /etc/network/interfaces.