First of all, I have moved this thread from blogging to web writing because an editorial calendar can be used in either a website or on a blog.
An editorial calendar is just a schedule and a reference for what you plan to publish in the future. The easiest way is to make one that will last through a year.
Open up a spreadsheet and make twelve sheets. Rename them Jan, Feb, March, etc. to December. Then depending on how you intend to publish there are a couple of ways to go about it. If you plan to publish daily, number the rows according to the number of days in the month. January, 1 to 31, February 1 to 28 (unless it's leap year), etc. If you plan to publish weekly, number each row week one, week two, week three, week four. If you plan to publish monthly, you won't need to number the rows at all.
Then jot down your ideas. For instance, you might want to do something special for holidays. Add your holiday ideas to the appropriate spots on your calendar.
You may want to use the spreadsheet columns for blog categories or website navigation areas.
You don't need to fill in the whole thing as one sitting. Plan as far ahead as you like. When you get an idea for an update or new topic, just add it to the appropriate spot.
At any rate, you can make an editorial calendar as simple or as complex as you like.