First download the modding tools and follow the instructions here: https://github.com/BrandenEK/BlasII.ModdingTools. Then download this modding API and any other mods you want to play with and extract them into the Modding folder.
To develop a mod for Blasphemous 2, run these commands to create a new template project:
dotnet new install Blasphemous.Modding.Templates
dotnet new blas2mod -n ProjectName -m ModName -au AuthorName -ve GameVersion
For example, to create a mod that adds the Boots of Pleading item, I would run the command:
dotnet new blas2mod -n BlasII.BootsOfPleading -m "Boots of Pleading" -au Damocles -ve 1.0.5