Once you are in the event editor, the task at hand is to draw the envelope. For a volume fade-in, all you have to do is draw a straight line that goes from 0 to, say, 75%. If you use the "Draw" icon aka the pencil using the right mouse button (for straight lines), it should be relatively pain free.
Don't get confused by automation in FL Studio. You have automation events for patterns (described here) and automation clips which are not linked to patterns (not discussed here). Two different things.
The video below pretty much sums up what I am trying to say above.
When drawing the envelope in the event director, you have to make sure that the level past the pattern you are currently considering is at the default level. You can see this in the video below.
This third video shows you how to fade out a tune using the event editor of the tune's last pattern automating the mixer master volume.
Personally, I find the event editor to be pretty horrible. First of, when you select a zone, you should be able to draw whatever you want within that zone and that should not affect anything else. Unfortunately, it doesn't work that way (the strange thing is that it does behave that way when you use the LFO tool). Also, whatever you do for one pattern should never affect what's after in the playlist but it does. For some reason, the event editor lets you edit the envelope past the last bar of the pattern. That doesn't make sense to me.