If you don't have an experience using the command line then it's a tad more involved then I can explain in-depth on mobile. Best I can do is give a brief outline.

To start with, wescode/lemmy_migrate is a python 3 script. If you are running windows install WSL (Ubuntu), once you have a command line I am familiar with you will want to download the repository from GitHub to a directory.

You will then need to create a config file called migrate.conf Use the sample provided in the repo under configuration. Edit it to use your information. You can use nano as a text editor.

Then it looks like the command would be something like:

python lemmy_migrate -c ./migrate.conf

Sorry if that is crap help, but I'm not near my computer right now, and don't often use Windows anymore to boot.

PS:

WSL is a program from Microsoft that gives you a mostly functional Linux command line within Windows. None of this is as complicated as it sounds, I'm using more words then strictly necessary to explain things somewhat at beginner level. The most time consuming part of this would be first installing WSL and then installing Ubuntu onto WSL. There are plenty of tutorials on how to do so.

Hopefully someone more familiar with Windows can tell you how to do the same thing from either the DOS prompt or from Windows PowerShell. It's doable, (almost anything is) I'm just not familiar enough with either to walk you through it.