Rule 1: not more than 20pp.
Rule 2: say ONE thing. That's all you have time for.
Rule 3: start with the CHILD aspect.
Rule 4: then consider EARTH.
Rule 5: HEAVEN should take care of itself.
Rule 6: FORGET the word “sample.”
(See my previous posts on this topic for more on the ikebana approach of heaven, earth, and child.)
Rule 3: choose a part of your diss. that really goes “ting” like a receipt coming out of a cash register or the fake sparkle on a set of mouthwashed teeth in an ad. That way the hiring committee is attracted to you and sees your work as unique.
Rule 4: your sample should bristle with citations. Then the hiring committee sees the quality of your research.
Rule 5: the general context of the job application should take care of the big picture for them.
BUT it never hurts to add an intro paragraph that lays out the heaven, earth and child of your project in general and your sample in particular.
Rule 6: the writing you give them should be polished, complete and through-composed. NO ellipses and square brackets that say “in this section I invent a great new idea that I'm not going to explain...imagine an amazing argument here...etc.”