Step-by-Step Guide

Development Environment

We begin with installing Visual Studio as our IDE. Download the installer and run it on your Computer.
Inside the visual studio installer, ensure that .NET Desktop Development and .NET Core Cross-Platform Development are installed, then click on the Individual Components tab and select .NET Framework 4.6.2:
Fork our ModStub repository using github. You can also use the template function of github to create a new, clean repo under your account. You should have your own copy of the JotunnModStub now. To get your link to the repo click on Code on the front page of your repo and copy the link in it.
Open Visual Studio and create a new project with Clone a repository. Paste the URL to your newly created repository on github and select a folder on your local hard drive for it.

BepInEx

To run or build a mod BepInEx must installed. Please use the BepInEx pack from Thunderstore as it's configured for Valheim, a release from Github may not work properly.

Customising the ModStub project

Once you have your base project, select the solution in the solution explorer, hit F2 to rename the solution as required. Rename your plugin project, an all namespace references, then right click your project settings and ensure the assembly name has also been changed.
Rename the PluginGUID PluginName, and PluginVersion to match your intended base release metadata. Your PluginGUID should contain your github username/organisation.
Your project base is now ready for use! You can proceed to step 3 to activate the Jötunn PreBuild tasks which add references, publicized dlls to your project. Or select a specific Tutorial to learn more about Jötunn and mod development for Valheim.

Optional: Use the renaming script

Inside your solution folder is a PowerShell script called RenameSolution.ps1. Right-click on it and select 'Run with Powershell' from the context menu. Follow the on-screen instructions to customise your project automatically.

PreBuild Automations

Add project references

Jötunn automatically references all dependencies that may be needed for the development process. To overwrite paths, create a new file called Environment.props at the project root and place the following contents inside, modify as needed.

<?xml version="1.0" encoding="utf-8"?>
<Project ToolsVersion="Current" xmlns="http://schemas.microsoft.com/developer/msbuild/2003">
  <PropertyGroup>
    <!-- Valheim install folder. This is usually found automatically, uncomment to overwrite.
         Needs to be your path to the base Valheim folder. -->
    <!-- <VALHEIM_INSTALL>X:\PathToYourSteamLibary\steamapps\common\Valheim</VALHEIM_INSTALL>-->

    <!-- BepInEx install folder. This is usually found automatically, uncomment to overwrite.
         Note the default value $(VALHEIM_INSTALL)\BepInEx requires the VALHEIM_INSTALL property to be set. -->
    <!-- <BEPINEX_PATH>$(VALHEIM_INSTALL)\BepInEx</BEPINEX_PATH>-->

    <!-- This is the folder where your build gets copied to when using the post-build automations.
         Note the default value $(VALHEIM_INSTALL)\BepInEx\plugins requires the VALHEIM_INSTALL property to be set. -->
    <!-- <MOD_DEPLOYPATH>$(VALHEIM_INSTALL)\BepInEx\plugins</MOD_DEPLOYPATH>-->
  </PropertyGroup>
</Project>

Warning

This prebuild task will add references to your current project. If you already have setup that references it most certainly will duplicate them.

Publicize Valheim assemblies

Jötunn can automatically create the publicised assemblies for you. To use this feature you must have the Environment.props file from the last step. Additionally you need to create a new file called DoPrebuild.props in your solution directory. If you are using the ModStub the file will already be there so just change ExecutePrebuild to true. If you opt not to utilise this automation, you will need to generate your publicised assemblies, and add them to your projects references manually.

<?xml version="1.0" encoding="utf-8"?>
<Project ToolsVersion="Current" xmlns="http://schemas.microsoft.com/developer/msbuild/2003">
  <PropertyGroup>
    <ExecutePrebuild>true</ExecutePrebuild>
  </PropertyGroup>
</Project>

Warning

This prebuild task will automate the generation of publicising the game assemblies. By enabling this, you understand that you will be generating new publicised assemblies upon PreBuild IF the binaries have been updated since the last time the PreBuild has run.

Manual setup

If you decide to disable the prebuild, you will need to:

Publicized assemblies are manually generated with AssemblyPublicizer. Please follow the instructions found on GitHub using the Valheim assembly DLLs found under Valheim/valheim_Data/Managed.
Reference those publicized assemblies and any of the Unity assemblies you want to use (at the very least UnityEngine.dll) in your mod project.

PostBuild Automations

Included in the ModStub and ModExample repos is a PowerShell script publish.ps1. The script is referenced in the project file as a post build event. Depending on the chosen configuration in Visual Studio the script executes the following actions.

Building Debug

The compiled dll file for this project is copied to <ValheimDir>\BepInEx\plugins or the path you configured in your Environment.props file.
A .mdb file is generated for the compiled project dll and copied to <ValheimDir>\BepInEx\plugins or the path you configured in your Environment.props file.

Building Release

The README.md in SolutionDir/ProjectDir/README.md is copied to SolutionDir/ProjectDir/package/README.md for packaging.
The compiled binary is placed inside of SolutionDir/ProjectDir/package/plugins
The contents of SolutionDir/ProjectDir/package/* is archived into a zip, ready for thunderstore upload.

Debugging with Visual Studio

Please see: this article