This is an easy to do project. It is also quite useful and if you have kids around, they will love it.
Things you will need:
Step 1: Take the Chosen Door Out
To make the following steps easier, take the door out. But it is also possible to do it with the door still in place.
Step 2: Remove All the Hardware
Next you remove all the hardware, wich makes sanding and painting a little easier. You could also just tape it off.
Step 3: Sand the Door
Just sand it lightly. I used 120 grid sandpaper and went quite fast over it. Theres no need for a sander, it wouldnt be much more work doing it by hand.
Step 4: Paint the Door With Chalkboard-paint
Just follow the instructions on your paint can. Mine said: light sanding and two coats of paint.
Step 5: Put the Hardware Back on and the Door Back in Place
Now just undo, what you did in step 1 and 2.
Step 6: Have Fun With Your New Chalkboard
This is where the fun part starts. Espacially if you have kids around. Your chalkboard door may even prevent them from drawing on your walls :D or the oppsite, youll have to find out yourself :)
You could use it as a groceries list or, thats what we use it for, a central place in the house to write things down we tend to forget. Like a to-do list.