To make the flat ones is a more difficult task than making the round ones. Flat ones are best made with a device that holds the handle, the straws, and the string for you.

Here is how I 've made brooms. Gather up the straw, untrimmed. You should experiment with whether you want to use the straw individually or as it grew, still attached to the thicker stems (not the full stalk). Seed can be still attached, or combed and rapped off--your choice. A little moisture content will allow the straws to bend and compress under the string so that you can really tighten it. You can soak the straw if it is too brittle to work with.

Place the thick end of the straws at the bottom of your wood handle. It will look like you are putting the straw on upside down. Four inches from the bottom of the handle, tie string as tight as possible to hold straw to handle. Tight! (You can tie one bundle of straw on one side of the handle this way and then tie another bundle on the other side, if you like.)

Once all your straw is tied on four inches from the bottom of the handle, tie it all again about one inch from the bottom
of the handle. Tight!
Hope this diagram works. #### will be the handle. --- will be the straw. ] will be the string.

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Now that you have your straw on upside down, you will have to bend it downward. Fold it against the four inch string place. When all the straw is bent down, it will be covering the places where it is tied on. Now, tie the straw down in at least two places again.

Once the straw is very secure, you can trim the ends even. I'd just use utility scissors.

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Hope this makes some sense. I learned broommaking from a 140 year old library book, rather than from another person. Always makes me sorry to see how many books get thrown away from our public library.