Alice, I'm a federal employee (National Park Service ranger). If it's the standard "171" form, just three important bits of advice here.
First, under Knowledge, Skills and Abilities (KSA's), take the form, cover it with white paper, except for the heading at the top, where it say's "Knowledge, Skills and Abilities" and xerox several copies. You can, and should, use several pages, because this section is where you really shine. No modesty allowed! Even if you've mentioned something you've done, or a skill you have, in an earlier section of the application, they will not credit you with the skill in the KSA's unless you mention it there, too. They won't say, "well, she mentions this in an earlier part, but she doesn't mention it in KSA's, but since we know she accomplished this, we'll give this credit as a KSA as well." So it's very importnt to list things in every section it applies, even if you're repeating yourself.
   Secondly, use active verbs. Rather than writing that you "helped"or "assisted" etc., write that you "facilitated" or "accomplished." This is very important, because you will get a higher rating if you use active buzzwords. Tese buzzwords are standard in federal applications, so much so, there have been lists of buzzwords written to help people with their applications.
Third, take nothing in the application for granted. If the standard form asks you what type of machinery you can operate, mentioned the kind of computer and typing skills you have, even if that sounds too elementary to you.

Feel free to PM me if you'd like clarification on anything I mentioned, or for any questions you might have. Good luck!

Chanteyranger