The port forwarding setting varies from different brand and model of router.
Here is a website which has the instruction to do port forwarding for almost all brands and models of router in the market.
We will take one TP link router as example.
1. Login to the TP-Link router via the default gateway address. Router's address is computer's gateway. If you don't know how to find computer's gateway, please google it.
2. Click on “Forwarding” and then “Virtual Servers.”
3. Click “Add New.” Enter the Service Port, Internal Port and IP Address. Then select “All” under Protocol and Click “Save.”
Usually, IP is DVR, NVR or IP camera's IP
Port is their http port.
After you finish port forwarding, you can open www.canyouseeme.org to check if port is open.