The SAQ is the Supplementary Application Questionnaire for the University of Cambridge, not Oxford! An additional piece of work may seem daunting for applicants, which is why it is a good idea to prepare for the SAQ in advance!
Perhaps the most important part of this application is the additional personal statement. Essentially, this is a very short additional information box, where applicants are offered more writing space as a part of their application. This box only allows for 1200 characters, which is enough for a short few sentences. Of course, you really should use this extra space; if nothing else, the university will be expecting you to use it! Think about what you could write in advance; perhaps type it out on a document beforehand, considering things like why you like Cambridge, or why you like the structure of the course, if these were not things that you were able to put into your actual personal statement. A good idea is to mention the nature of the collegiate living environment, or else the existence of one on one tutorials that the university provides.
Another important part of the SAQ is the photo submission! If you can, try to have a passport, or similiar, photo ready to send in! The listed requirements of the photo are:
(a) be in portrait orientation (eg 840px high by 700px wide) (b) be in colour (c) have a plain light-coloured background (d) show your head and the top of your shoulders only, with your face central on the picture (e) have good colour definition, not too dark or light (f) be in focus (g) show your face un-obscured (no sunglasses, hat or scarf) unless you wear glasses or cover your hair for religious reasons (h) show you acting naturally, not smoking, not with other people or in a 'holiday snap'.
Clearly, there is no mention here of a neutral expression, which may be why you would like a photo different to your passport photo to be sent in. This photo may be revealed in the future if your application is successful, so be warned!
The third thing that I suggest, is be ready for extremely thorough questions regarding your past and future examinations, along with your grades. This will include exact UMS scores at least for AS levels or equivalent, so make sure that you have these on hand. Other than this, you will need to be aware of your past educational body's names and addresses, as well as the time periods that you spent in each one.
The SAQ is not something to fear, though it is something to be taken extremely carefully, so, whatever you do, do not rush filling it in!