It's easier on some bikes than others due to various combinations of rim & tire features.

This is my favorite flat-repair video.

As a safety check, when you finish, you can do 2 things: firstly, push the valve up into the well of the rim and then pull it down again to make sure the tube isn't trapped or twisted there. Secondly, you can make sure the tube is seated inside the tire properly and not pinched by the tire bead by pushing the tire bead aside and making sure you can't see any innertube, just your rim strip. Example in this video.

Now that you have the original tube back in your tire, did you ever figure out why it went flat in the first place? Did you just not fill it for a long time, or did it apparently have a slow leak?