At least 3 seasons of good CL performances, more likely 5. We get a coefficient each year depending mainly on how well we've done in Europe, plus there's a national one that all teams get. The last 5 years get added together to give your total coefficient.
Our last 5 (oldest to newest) are 20, 3.6, 16.7, 20.1 & 10.3 giving a total of 70.7 (I've rounded up for simplicity).
Next season we drop our best coefficient but the year after we drop our worst. If we get 26 in each of the nest 2 seasons we'll have just under 100 in 2 seasons time, which would be top of pot 2.