The steps required depends on which router you have. For example with some D-link routers the following 5 steps would be used:
1.Fire up your D-Link router’s Web configuration screen and click on its Setup tab.
2. From there, click on Network Settings on the sidebar.
3. Scroll down to the Add DHCP Reservation section, check the Enable box and add your device’s name.
4. Add your device’s IP Address and MAC Address. The IP Address you choose will need to fall within your router’s DHCP UP Address Range (see the DHCP Server Settings section directly above). You should be able to locate your devices MAC Address on the bottom of the device itself.
5. Click Save.
How to Set Up a Static IP Address from Your Router | D-Link Blog