Plan, plan, plan...
Even if you have already started writing, If, like me, you spent your first writing session watching the cursor blink on an empty word document, there are plenty of places you can get the ideas from. The Student Room's personal statement builder is a great way to gage exactly what you should cover, and their archive of statements are sorted by subject and by university, so if you are applying for an English Language course at Cardiff Univeristy, you can find an archive of what your chosen universities are looking for. The UCAS website also have a few helpful videos that are perfect for watching during tea/chocolate/crying breaks.
...even if you've started
Hitting pause and writing a plan of action can allow you to take a step back, to figure out excactly what you want to say, and where you want to say, overall makimg the process a lot less stressful, and it can put the dozens of ideas circling your mind onto paper straight away.
Don't overthink it
As you may of guessed at the end of my first tip, yes, my application has sent me on a bit of am emotional roller coaster; with considerably more lows than highs. It's easy to get caught up in every detail and every piece of advice you may recieve through family, friends or the Internet. But to avoid going into overdrive, just remind yourself of why you want to go to said university, to said course, and you are half way there.
Get a second opinion
You've finished your first draft; and its like a weight has been lifted from your shoulders. But then you begin to have second thoughts: is it actually any good? Getting someone to read, edit, chop and add to your draft - be it your tutor, friends at university or your parents aka people that have seen or been in your situation before, will create the balance of your passion and excitement, with their wisdom and experience.
Draft one, two, three
Drafts are your best friend. It's tempting to want to finish the entire application by spending a whole day tiring over it, or even pulling an all-nighter to add in those last few of the 4000 characters, but taking it step by step gives you time to recollect thoughts and collect advice from others.
And lastly, just do it!
It's as much work to procrastinate and worry about starting your statement, than actually starting it. Just stay calm, stay true to yourself, and stock up on extra strong tea and coffee supplies.