Search your steam library for "valheim", the dedicated server will appear, install that through the library interface. After that, right click that in your library list, and go to manage>browse local files. Once you have that directory opened, find the PDF file named "Valheim Dedicated Server Manual" and open that, give it a good read and don't skip anything, especially the part telling how to set a password in the files, if you want the server to be private.

To port forward easily, just press windows key and R to open the "Run" in the search field, type "cmd" without quotation marks, then press enter. You will see Windows Command Prompt appear on screen.

Type "ipconfig" without quotation marks into cmd, press enter. It will show you your ip configuration information.

Find the IPV4 address and write it down somewhere safe, we need that to port-forward your server. Find the "Default Gateway" and type the number it has shown into your web browser url address bar, and then find your login information on the back of your router, and sign in.

Opening port ranges for Valheim

Each router will look slightly different, but all provide general purpose firewall management and LAN settings. We're looking for "Port-Forwarding" or something along those lines. Add a new port, range using your IPV4 address as the IP address, and port ranges 2456 - 2458 set your protocol to "UDP/TCP" or "Both" and save.

Some routers let you disable and enable ports by a toggle button, so return to the port-forwarding list and ensure that it is enabled after you add it to the list of ports. Now click save somewhere on that page, if there is an additional save button at the bottom or top of the port list. If not, continue on to your steam library and run the server.

Connecting to the server

Find the IPV4 (the ip you will use to connect to your own server, if running it on your own network.) In Steam, click View in the upper left-hand corner, then servers. Go to favorites and click "Add a server" enter your IPV4 address and the port Steam will need is 2457.
So for example "IPV4address:2457" then save and refresh while you have the dedicated server running on steam. Connect to it and type in the password, if made to be a private server. The game should launch, and it may ask for the password again once in-game connection begins.

Others connecting to your server

Search "what is my IP" into google, and it will show you the address others will use to connect to your server, provide that in the form of yourIPaddress:2457 and tell them to add it to their Steam server favorites, and connect through that window.

You may also connect through the in-game IP connection button, but you will have to manually type the address each time, and you will be using the port 2456 if connecting through the game only, not using Steams server favorites list.