You can claim and use 1 Resource Token per day by visiting the Power Tower and clicking 'Free Token' then clicking again on the token that appears below. This will give you either free gold or rubies and resets at 12am UK time.
For this achievement you must do so 20 times, and the 20 days do NOT have to be consecutive. There are also achievements for using resource tokens 50 and 100 times. Happy dueling