A personal statement should reflect several things: namely, your passion and suitability for the field(s) in question, as well as your commitment to study (a degree is obviously a commitment of several years of your time, so university admissions staff consciously filter out applicants who don’t seem likely to follow their studies through to completion).
In order to write the best possible personal statement, I would advise the following process:
1. Brainstorming: this process shouldn’t be rushed – you need to give yourself enough time to think about what it is exactly that draws you to your field(s) of choice, as well as elements of your life course so far that indicate your suitability and/or ability for the field(s) in question. You can search online for personal statements from previous years. Although this can be helpful, please remember not to plagiarise (copy others) – UCAS will be able to detect this. If you don’t trust yourself not to copy someone else’s ideas, then perhaps it is better to look through examples of other’s statements when you are further along in the process (e.g. after step 4).
2. Speak to others: as part of the brainstorming process, don’t hesitate to speak to trusted others about your reasons for choosing a particular degree (e.g. your parents, grandparents, teachers, etc.). Preferably people who know you well and will be able to help you identify your key strengths and abilities. N.B. the more people with whom you speak, the clearer things should become – however, don’t stress if you feel a bit muddled at this stage, I would even go so far as to say that feeling a bit muddled indicates that you have multiple reasons for your choice and a depth of interest, which are good things!
3. Planning: start organising the points that you came up with in steps 1 and 2. The idea here is to start grouping them into paragraphs. Remember, the flow of your writing is very important. Admissions officers have thousands of statements to read in the selection process, so you want to make their job as easy as possible. You've probably heard it before, but the introduction and conclusion are particularly important. (If you're curious as to why, then check out this link: https://rapidbi.com/primacy-and-recency-effects-in-learning/).
4. First draft: start writing! Don’t put too much pressure on yourself. Your first draft is going to be far from perfect – just start writing and you’ll be surprised at where your words will take you. I’ve often found that I resist starting the writing process but once I start I can’t stop (and then need to cut my words back to fit the word limit!)
5. Show your first draft to trusted others: try to choose someone who has the right balance between being encouraging and helping you improve.
6. Revise your statement until you are happy with it.
7. Check that your spelling and grammar is perfect: a simple error in spelling, syntax, or grammar could be enough to put admissions officers off – so you don’t want to take any chances. Use a spellchecker on your word processer (make sure it is set to UK English. US spelling is not acceptable to admissions officers.)
8. Preferably: there’s still some time left after steps 1-7 and you can put your personal statement away for a week. Time and distance can often bring a lot of insight and give you a greater ability to critique your own work.
9. Submit: make sure you leave plenty of time for this, in case something goes wrong (like your computer playing up). Aim to submit your application AT LEAST 72 hours in advance of the deadline.
10. Wait: to hear back from your future college/university :-)