Joel Leighton put NWLJ ahead shortly after the break with some great individual play, taking the ball past one defender and slotting the ball round the Letchmore goalkeeper.
Leighton was on the hunt for a brace and was rewarded for his efforts courtesy of a marvellous strike into the far corner of the net eight minutes from time.
Letchmore pulled one back with two minutes to go, from a free kick which was whipped in from the left and headed in at the far post.
The Jets held on for the win and look forward to the cup final next week.