My career is in recruitment, I have years of experience writing job descriptions, interviewing etc so I can give you some pointers.
When I read through applications I find candidates usually give too little information or too much, it's important to stick to the points you're making. Also for popular vacancies (such as an admin assistant) the number of applications are likely to be extremely high, so you need to ensure that your application is one that sticks out from all the others - it helps to be precise but concise, reiterating the point that you're the perfect candidate.
Firstly, have a thorough read through the job description, familiarise yourself with what the daily tasks will be and also some info about the company etc will help you focus on the job - you know exactly what they expect their new employee to do.
Secondly read the person specification thoroughly. Have a look at each criteria - for example: if it's 'essential' or 'desirable'. Some indicate how they will test the criteria e.g. using 'application', 'interview', 'assessment' etc.
Highlight every criteria to be measured at the application stage (or all of it if it only says essential/desirable/ideal) and look at each one individually. You need to show the recruiter you can 'meet and demonstrate' everything they're asking on the person specification so write down an example for each point. You can also relate to any education/voluntary experience too. Think of the best examples you can, and keep it to the point.
Some typical criteria is:
(1) - Ability to negotiate/prioritise tasks - effective time management skills, initiative, pro-active, multi tasking skills, flexibility (e.g. working late to meet a deadline) - team working skills/being able to work independently
(2) IT skills - software packages/databases/MS office - where have you used these and in what capacity? How long? Any IT qualifications (e.g. RSA)
(3) Office experience - worked in a fast paced environment? How well do you thrive under pressure?
(4) Customer service/polite talking manner - being able to handle difficult situations
Once you have made your notes, you can start to compile your statement but just make sure it flows freely and makes sense. You can also follow the order on the person specification if it helps. Your opening statement should highlight why you're applying for the job, what interested you (e.g. wanting to work for a prestigious company/career path/passion for a certain field) and move on to discuss your experience and job skills. (E.g. my interest in this position stems from my experience in my previous role with... I am looking to work in this field and expand my career, I feel this would be the perfect opportunity for me to do this.)
At the end reiterate your interest in the role and thank the recruiter for reading through your application - I would say that you should keep the statement between 1 - 2 pages (1.5 ideally). Make sure you're honest though - you could be asked something at a later stage and forgot what you wrote in your application! And claim your achievements
Don't be afraid to write "I" - this will show that you have confidence - believe in yourself. So many candidates I have interviewed in the past shy away from using "I" as they are too scared to they will come across as arrogant or over confident...so they use "we"...."We did this, we looked at this." - If the example you're referring to was a team effort that's fine, but again if it was something you did personally - do claim it and be proud of it!
Once it's all written up check through for spelling mistakes, grammatical errors, punctuation etc.
The important thing is to keep it concise, to the point, show that you meet the criteria and provide good examples. Mention your skills, and passions!
Good luck - if you need any more help don't hesitate to ask
