This is so easy you could almost guess it!
The best count period is during mid-late April or early May (although all information is great). View from a distance and be aware that sitting birds or those in a hollow in a field can be difficult to see. Crows or other disturbance nearby often cause these birds to fly up making them easy to count in the air. We need to know the number of pairs, so count 'em all on your site and divide by two :-), the more visits you make in this period the more accurate your estimate will be. If you see birds sitting on nests or with chicks please also include this information as this is positive proof of breeding and use of a specific field or site.
Full BTO-style surveys have slightly more stringent guidelines attached and for these the following reference should help:
Gilbert G, Gibbons DW & Evans J, 1998
Bird monitoring methods. A manual of techniques for key UK species.
RSPB
but for our purposes keeping things simple should produce more results.
Last updated 09 February 2002.