The first legato technique we're going to look at are hammer ons. The name "hammer ons" is somewhat self-explanatory. This technique is going to involve our fingers hammering on the fretboard in order to produce notes.
In order to play a hammer on, what will usually happen is you'll pick a note, then after that, a finger on your fret-hand will snap on a different note. So two notes are played, but only the first note is picked. The 2nd note is produced solely by your fret-hand finger.
Now, notice how I said "snap" in the previous paragraph? The reason I said that, is because your finger needs to have a good amount of force behind it in order to get the note to sound properly. The goal with all legato notes is to make sure they're all equal in volume. As I mentioned in the video, think of hammer ons like you would an actual hammer. You wouldn't just gently place a hammer on a nail would you? Nope, you need to give it some force. In the same way, you can't just have your finger gently fall on the note, you need to snap it on there.
Practice the exercises tabbed out below. Then, when you have the feel down for hammer ons, let's move on to pull offs.