1) Install and configure sshd on your server. If you can log in via SSH on your local network you're good, just make sure that you configure it to be as secure as possible. Use non-default ports, disable guest/anonymous logins, etc. More can be found here:
2) Assign a static IP to your server, either on the server itself, or by setting up a reservation on your router. If you assign the IP manually on your server, be sure to exclude that IP address from the DHCP pool on your router so it doesn't get handed out to another device. This IP is an internal IP address in the same subnet as every other device on your network, you just want to make sure that once you forward the ports they will always point to the right device.
3) Forward the SSH port to your server's static IP address. The default port for SSH is 22. The method for forwarding ports on your router is also dependent on the type of router you have.
4) I recommend, if your router supports it, signing up for a Dynamic DNS name on something like dyndns.org, and then configuring your router to automatically update whatever hostname you pick. That way if you're out away from the house and you want to log into your server, you can do so using a hostname like yourserver.homeip.net instead of having to worry about figuring out what your public IP is, and checking to see if it's changed recently.