Daily active users is the number of unique people who showed up in a single 24 hour period. Monthly active users is the number of unique people who showed up at least once that month.
To get the ratio of DAU to MAU, you can either take the average DAU for each day of the month, and then compare that to the MAU, or you can take the DAU to MAU ratio for each day and then average that. They'll both come out about the same (doing it the second way smoothes it out just a bit more because of the loss of precision which makes the month to month comparisons or quarter to quarter a little smoother).
To get the ratio of DAU to MAU, you can either take the average DAU for each day of the month, and then compare that to the MAU, or you can take the DAU to MAU ratio for each day and then average that. They'll both come out about the same (doing it the second way smoothes it out just a bit more because of the loss of precision which makes the month to month comparisons or quarter to quarter a little smoother).