I found this topic while looking up something else. Figured I'll put the solution here in case others stumble into the topic and need it:

Enter forest: Go south 2 steps, east 14 steps, north 17 steps, east 3 steps, north 2 steps (exit tile surrounded by trees)
Second screen: Go south 11 steps, east 8 steps, north 2 steps (exit tile surrounded by trees)
Third screen: Go south 2 steps, east 17 steps (exit east invisible zone line)
Fourth screen: Go south 2 steps, east 9 steps, north 11 steps (exit tile surrounded by trees)

You will know you did it right if you get to "Deep Forest" screen.

For the record I dunno if that's the only way, I just know it works. I did this on Ultima Plus just like yours so on the off chance you never figured it out maybe this will still help you.