Make sure that you fill in the application carefully; if it is not complete, it might not be taken into consideration. The employer is likely to receive hundreds of applications. Try to stand out positively from the rest.
A covering letter should not be longer than one page. Describe briefly why you would be the best candidate. Name one or two referees, and give their contact information; the employer may be interested in calling them (possibly even before an interview) to ask what kind of employee you are. Remember to sign the letter.
The length of the CV should not exceed two A4 sheets.
Before making a spontaneous application, look at the company's website to see what kind of career opportunities you can expect and how the employer usually hires new staff If there is an electronic application form for spontaneous applications, use it. If not, contact the employer by e-mail or phone. If you make the first contact by e-mail, call the company after approximately 1 week and ask whether the people in charge of recruitment have received your application and had time to consider it.
In smaller companies especially, there may not be enough staff to advertise vacancies, handle applications, organise interviews, etc. Therefore, it is worth contacting the employer directly and applying for a post spontaneously.