Step 3 - A "G" from an "E"
Now that we've tuned our "A" string from a note we got from the "E" string, we're going to tune out "G" string, also from a note we'll get from the "E" string. We're going to tune all of the strings relative to the "E" string, for this simple reason: any "inaccuracies" in our tuning will not be carried over from string to string.
Press the "E" string (the one you just tuned) against the 3th fret, and pluck it. Guess what note that is? It's a "G"! Go ahead and tune your "G" string to the note your "E" string plays on the third fret. You have to sort of "carry" the note over to the "G" string, you can try humming the note and see if that helps. You should remember from the beginner's lessons which string is the "G", and how to tune it.
Try to think about what you're doing while you do it: "E string, 3rd fret, G string..." to help you remember later.
Before you move on, spend a little while de-tuning and re-tuning the "G" string. If your ukulele is one of those brand new inexpensive ones that likes to go out of tune all the time, you might want to start again back at your "E" - if you get frustrated, have faith that the thing will eventually settle in... and you may have become proficient at tuning it by the time it does!
Congratulations, you've finished Step 3!