step 1.

download the latest mod version from the versions page

step 2.

install modrinth

step 3.

create a new minecraft instance on modrinth. the version should be 1.21.4 and modloader should be neoforge

step 4.

in the instance you have just created, select "add from file" from the dropdown (ｖ) at the bottom

step 5.

select the ".jar" file you downloaded at the start then press play