The Baseline data is automatically populated when you save a baseline. Project ribbon > Schedule Group > Set Baseline.

When you save a baseline Project will copy:
Start --> Baseline Start
Finish --> Baseline Finish
Duration--> Baseline Duration
Cost --> Baseline Cost
Work --> Baseline Work

for each task. You can then view variance by applying the Variance table or if you are using the Tracking Gantt you can see the baseline bars as compared to Current.

I hope this helps.

Julie