The rule is to follow instructions closely. The employer may provide a phone number or e-mail address for you to obtain more information about the vacancy. Do not use this first contact as an opportunity to present yourself if this has not been requested.
It is quite common for employers to accept a limited number of applications, and to close the vacancy once that number has been reached, so make sure your application is sent in good time, and always before the closing date.
When completing an application form, be sure to save it and send it in a compatible file format. Include the vacancy reference number and proofread the form carefully. Spell-check the form using a British rather than an American dictionary and pay close attention to grammar and sentence structure.
If the application procedure calls for a CV and a covering letter, keep the letter brief (one side of an A4 paper) and factual. Say why you are writing in the first paragraph; give details of your suitable skills and experience in the second paragraph; give your availability for an interview in the third paragraph and any other important details about existing commitments; finish with a short sentence to say that you are looking forward to their reply. Try to include key words from the advertisement in your application.