This apparently.
"How will the play-off draw work?
The play-off draw will be held on January 31 at the House of European Football in Nyon, Switzerland, and will be streamed live on UEFA.com.
Clubs that finished ninth to 16th in the league are seeded in four pairs (9 and 10, 11 and 12 and so on) and the rest will make up four unseeded pairs (17 and 18, 19 and 20 etc).
Each seeded pair will be drawn against an unseeded pair. For example, nine or 10 will play 23 or 24, while the teams that finished 11th or 12th will come up against the teams that were 21st or 22nd in the league. The aim is for seeded clubs to play their return legs at home.
The draw allocates each team from a pair to one of the pre-determined positions in the bracket. For example, for the first draw, one of the clubs ranked 23rd or 24th will be drawn in one side of the bracket, leaving the other to be automatically placed on the opposite side. The unseeded pairings will be drawn first, with the process then repeated for the seeded pairings, starting from the teams ranked 15 and 16.
Teams can be drawn against another from the same country or an opponent they faced during the league phase. The eight teams that come through the play-off phase will then progress to the last 16. The top two seeds (currently Liverpool and Barcelona) cannot play each other until the final."