Final thing you need to memorize before you can use the doomsday algorithm are the actual "doomsday dates". These are the dates across the year that fall on the same weekday. Of course you can add or subtract any multiples of seven if you feel you have better mnemonics for any particular month.
The first two, January and February are dependant whether the year is a leap year or bot. So you need to figure out that first. But you should now know how to do that. For January the doomsday is 3 on regular years and 4 on a leap year. Like there are (usually) 3 regular years and the 4th one is a leap year. February's doomsday is the last day, so 28 or 29 depending if it's a leap year or not.
On match the doomsday is 3, the pi day (3.14). All the remaining even months have the same doomsday as the month number. For example April, as a fourth month has a doomsday 4.
The remaining four month's dates can be figured out by remembering the phrase "I work from 9 to 5 at 7-11". The doomsday is the other number in those number pairs from the month number. E.g. September is the 9th month of the year so the doomsday is 5.