How do I level (screed) the concrete over an area that is wider than my screed board.


You can tackle this several ways. The professionals will drive grade stakes inside the forms at convenient intervals (less than the width of the screed board). The grade stakes will have the proper level marked on the stake, usually a nail in a wooden stake. You can place some concrete in the forms and using the grade stakes as a guide, begin screeding the concrete. As you get the proper grade on the concrete, you can pull the grade stakes. As you can tell, this requires a little experience to get it just right, but beginners can do it with less accuracy.
Another method is to get a probe, such as a stick or steel stake, mark it with the depth you want the concrete to be, then place the concrete in the forms. Screed the concrete and probe to ensure the proper depth. This is not as efficient as the first method, but it will get the job done with a little more effort. You can set construction joints (1x4's, metal keyway forms, etc) inside the forms at intervals of 25-30 times the depth of the slab. You can use these as screed guides.
Finally, you can set screed guides inside the forms using 2x4's and stakes so that the bottom of the 2x4's are at the top of where you want the concrete to be. Nail ears on the end and top of your screed board so that when you set the screed board on the screed guides, the bottom of your screed board is even with the bottom of the screed guide, thus level with the top of where your concrete will be. Once you get your concrete down and screeded, you can pull these screed guides and fill in the stake holes and re-level by eye.