I saw something similar to this in a Joann Etc. store for $30(!), and I decided I could make my own for considerably cheaper. The frame is the only thing I had to buy and I got it at a dollar store - the rest of my materials were leftover school and craft supplies I had lying about, so the whole thing only cost me a dollar :)
Get your materials! Everything can be bought at a dollar store, although you probably have a lot of these materials hiding in a pile somewhere if you look around your house.
I used a piece of cardboard for a straight-edge to keep things from looking wonky, but if you like things to be really even you'll want a ruler.
Start off by using your straight edge or ruler to draw a line straight across the paper, just a bit down from the top. It should form a thin rectangle across the top - this will be where you write the month on your calendar.
Underneath the top rectangle, you'll use your straight edge to make a 5x7 grid for the days of the month.
Now, I used a post-it as a lazy-person substitute for a ruler. I basically estimated how big the squares should be and used the post-it to make them all about the same size. If you want to be REALLY neat about it, measure the width of the paper and divide by 7 to determine the width of the squares. Measure the height under the top rectangle and divide by 5 to determine how tall your squares should be.
You should now have a piece of paper that looks like a blank month from a calendar.
Note: There are a *few* months (like ones that start on a Saturday) that won't fit into a 5x7 grid. You should make another paper with a 6x7 grid to be prepared for any of these months (unless leaving a few days off your month doesn't bother you for some reason).
Put your paper into your lovely frame! Make sure that you have the paper oriented correctly so it isn't upside down when you try to set it up. If you made an additional 6x7 grid you can stick it in the back of the frame so you don't have to worry about losing it.
Use your dry erase marker to write in the month at the top and the numbers of the days in the boxes.
If you have colored dry erase markers, jazz up the calendar by highlighting the design of your paper with those dry erase markers, around where you wrote the month.
Start writing down all your appointments and social events! At the end of the month you can erase all your days of the week, events, and the name of the month and write in all your new exciting plans for the next month!