Decide how long a year in your calendar system will last. A typical earth year is 365 (and 1/4) days. Yours can be longer or shorter depending on what you are trying to accomplish. Ending your number of days in a round number will make it easier to divide the year later.
Determine how many "months" or smaller units will comprise your year. Invent a name for each of these months, and write these down. For example, to keep it familiar to the human calendar, you could use "Dekemviera" for December.
Divide the number of days in a year by the number of months in the year to calculate the number of days in a month. Depending on the number of days and months, this may not be an even number. On the earth calendar, not all months have 30 days, so you can add or subtract days from some of the months to make them all fit into the calendar.
Add up the number of days in each month and make sure they still total the number you came up with earlier. You can alter the number of days in the year at this point if you need to.
Decide how many days are in a week, and how many weeks are in a month. On your fictional calendar, a week may last for eight days (and be called something other than a week) and there may be three weeks in a month.
Make a grid using a ruler on a large sheet of paper. Make the grid big enough to accommodate a month in your calendar system. Place the number of weeks vertically and the number of days horizontally. Repeat for the number of months in your year.
Start writing the numbers of the days in the calendar you have created. You now have a graphical representation you can reference while plotting the calendar year for your year.
Decide which year in the epoch in which the events of your novel take place. On earth, we use the Gregorian calendar which counts from the birth of Christ.