First of all, you're going to need an emulator, probably for Android, if you want to rip from your computer.
Nox is my emulator of choice for ripping from FEH, for the following criteria: it can download the app from the Play Store, can actually play and interact with the app (which you'll need to do if you don't want to link to your FEH account), can enable and disable root mode, comes with a file manager, and has a shared folder where you can move files between your computer and the emulated phone. I'm using version 6.2.8.1.
Secondly, you'll need an image editor capable of converting or opening webp images. (This is very important for a reason you'll see later.) I use
GIMP 2.10.8 for my submissions.
Third, though this is only required for map characters: this
SSBP viewer will allow you to view animations for a map character and make icons for them. Extract the zip anywhere, but I personally recommend your downloads folder or desktop.
Open your emulator, go through setup, and install FEH. Make sure you do not have root enabled before you open FEH; As far as I know, FEH automatically blocks rooted devices from playing, though I don't know if this applies to the tutorial. Open FEH and link your account or play through the tutorial until you get to where you download additional data. There's a few gigabytes, so grab a snack or something and wait.