Try and relate it to the job description and personal spec, if there is one.
For example, if it says 'excellent keyboard skills required' and you don't have any certificate but do have the skill, you could put 'I have used keyboards for x amount of years, and whilst employed at Smith & Jones accurate and fast keyboard skills were essential' or 'My keyboard skills are adequate and I am very keen to improve'.
Mention anything that you have done in your spare time if it is relevant eg 'I enjoy working as part of a team and was a keen member of the school drama group for x years'.
Good luck