You need to add at least enough oxygen to meet your minimum oxygen requirements. If you haven't altered anything from the default that should be 0.1 atmospheres. Next you need to add enough gass that the oxygen is less than or equal to 30% of the total, so you need at least 0.2334 atmospheres of other gasses. As you need to warm it up as well you can use CO2 for this.
The exact order of doing this can vary if you are trying to optimise your colony crowth, eg on very cold/hot worlds it is better to warm it up/cool it down, until you drop to a colony cost close to 2.0 before adding the oxygen.