My sister makes her own dog beds, I've even bought all the material for a bed but have not made it yet!

Go to a fabric store and buy the fabric, the amount of fabric depends on the size of dog bed you are going to make.

You can always have someone at the store help you figure out how much you'll need. You'll also have to buy a long enough strip of velcro to close the end. (or a zipper if you are good at sewing)
The velcro or zipper are needed if you want to be able to take the cover off and wash it alone.
Also you'll need the filling, you can get stuff from a pet store, or you can buy Polly-fill at the fabric store. My sister uses two bags of polly fill for her bed for her husky.
(She throws the entire bed in the washer, polly-fill and all, so she didn't have to use velcro or a zipper.)

Take the fabric, and once you've deceided how big if a bed you want to make, turn it inside out, sew up open edges, leaving one open so you can sew in the velcro. Turn it right side out, fill with polly-fill, or what ever you are using (I think I am going to use an old blanket or two) Close the velcro, and you're done!