In your case, wait 150 days and see if it's done. It really is that simple, construction ship with the module + order, wait, done. You can even see it on the map, the JP that was supposed to be stabilised should have a square around itself (unlike the usual circle that planets or asteroids have). Do note however, that you have to do the same from the other side too, 1 JP navigable from both sides requires 2 stabilisations.