The first step of doing a fecal test is collecting the samples. The easiest is to go in the morning in the coop and collect a sample of different looking feces that are conveniently lined up under the roosts. Put them in a Ziploc bag and squish them together. Put the bag in another bag to avoid any leaks.
Second, drop off the samples at your vet, or send them directly to a lab.
We use the California Animal Health and Food Safety lab services (aka CAHFS) as they are cheap and fast (and we are in California). Download and fill out the submission form. Here is an example of the form to show what fields to fill out to request a simple fecal test. Put it with the sample in an envelope and ship it via FedEx Next Day Delivery. Call CAHFS beforehand to get their FedEx account number as they have a very good discount.
A couple of days later, you should receive the result of the fecal test.