First, launch the Facebook app on your phone. From here, you will select the "Check In" button at the top right.
If Location settings are turned on, you should see a few options of places that are near you to choose from.
Select the pace you are checking in to. Then you can add your own personal message to the check-in. You can tag friends or add any hashtag or image to the check-in as well.
Once you click "Post", you should see your check-in show up on your feed where friends can react and comment.