A Lebensohl auction takes place if the following conditions are met:
there was an opening: 1♣ or 1♦;
opponent has bid weak 2♦/2♥/2♠.
Bidding depends on the suits shown by the opponent (answers, if possible):
double - four major 9+ HCP or GF on the minor;
2♥/2♠:
in an undeclared suit - natural, non forcing, 5+;
in the suit of the opponent - 10+ HCP, four major or both majors as long as not reported or short;
2NT - weak with clubs or support or GF 4-5+ majors;
3♣/3♦/3♥/3♠:
in the suit of the opponent - 24+ HCP including partner, four major or both majors as long as not reported or short;
in an undeclared suit - 21-23 HCP including partner, invit, 5+;
3NT - to play.