It's pretty simple, the important part is the location. Find a stream with shallow fast running water and somewhat deep pools. Find a place where water from a pool narrows and becomes a fast stream. Start ~200ft (if the stream allows it) down stream and head towards the pool with a net in hand kicking over rocks. In my expirience 9 out of 10 times, little (3-6in) flatheads will swim upstream and hide under another rock. As long as you can keep an eye on them they are not very hard to net. The 200ft is to give enough space for muliple attempts at the same fish. When picking a location, if you know there are flatheads in the water, which in Iowa there should be plenty, look for shiner and other minows, because that is what the young flatheads eat. if there are shiner, there are young flatheads.
Good Luck