Most jobs will ask you to complete an application form.

The usefulness to an employer is obvious - it collects and organises all the key information needed from the applicant, and it makes it easier to make like-with-like comparisons between applicants.

And it can make life easier for the person applying.  The hard bit comes when you finish all the factual questions - name, address, education, work history - and come to the section with a title like: 'Statement in support of your application'.

A good approach is to focus on the job you are applying for - look carefully at the job specification which should accompany the application form.  What does it ask for?  And how does it match with your skills and experience?  Compose your response in three parts: first, state what it is you are claiming; secondly, provide the example or evidence from your experience; and finally comment on what you learnt or how you feel it applies to the prospective job. 

Applications look best if they are typed (and spellchecked).  If it is an online application, print it off and have a read through before you submit it. If it is a printed form, put a note in the box for Personal Statement to say that it is attached as a separate sheet - which you will have typed.