It’s fairly simple depending on what equipment you are using for your router/firewall but here are the basics.
The following are examples:
You have a range of public IP addresses avaiable from your ISP 188.8.131.52 – 184.108.40.206 and your internal IP address scheme is 192.168.0.0 class C 255.255.255.0.
Your work station that you want to open the web services to has a static IP address of 192.168.0.100 (note: must be static).
What you need now is to pick an available IP address from your public IP address and say we pick 220.127.116.11.
Now you must tell the router that any external requests to 18.104.22.168 must be redirected to 192.168.0.100.
After the mapping above has been done you must then tell the router/firewall what types of traffice you want to allow through to 192.168.0.100 such as HTTP or port 80, FTP, POP3, SMTP and so on.
Those are the principles and will not change from router to router or firewall to firewall.