Mark the cards you want to drill.
Choose "Drill marked" on the Deck menu.
Then type into the dialog the total number of cards you want in the drill.
For example:
If you mark 4 cards and type 25 cards in the dialog then, there is a 4 in 25 chance
that one of the marked cards will be displayed each time.
There is a 21 in 25 chance
that some other card in that deck will be displayed.
In short, drill marked increases the frequency that certain cards will be displayed.