Question on setting up limit switches

I have a openbuild machine and the builder didn't add the limit switches. I've watched a video on this and have a couple of questions.

To get the GRBL to know that the switches are installed, and react correctly when hitting one, do you JUST change $21 to 1, or are there other steps?

Once the switches are installed and GRBL knows that they are there, does the home feature automatically work?

I'm using Bcnc with a Raspberry Pi3.

Any direction would be appreciated.