Once you lock back the slide, Page 23 of the manual advises to pull the takedown lever to a 90 degree position and pull until you hear a click, I believe it's two clicks if I recall, but in any regard you do not want to remove the lever from the frame. Go easy.
You will see a cutout in the takedown lever and you want to line that up with the retaining spring in the frame, I want to say it's about 3/4 of a turn down, push the lever back in. On the one I owned it was likewise easiest to pull out and push in at a 3/4 turn up or down rather than at 90 degrees.
Once you get the takedown lever back in do not remove it from the frame. Use the first joint on your index finger as "leverage and travel stop" against the frame until you feel or hear two clicks which should help in preventing removal from the frame.