How does a reference look like?

The reference really should come from your referee, so if you are writing the whole thing just for them to sign its a bit naughty.

That said you could write a draft, which includes your best points and reminders about what you have studied and the skills you want to emphasise.

LSE and Stratclyde have some examples.
https://www.google.co.uk/url?sa=t&rct=j&q=&esrc=s&source=web&cd=6&ved=0ahUKEwjyjcOB57TNAhXDI8AKHQ4hC7AQFgg_MAU&url=http%3A%2F%2Fwww.lse.ac.uk%2Fintranet%2Fstudents%2FmoneyMatters%2FfinancialSupport%2FinformationForReferees%2FGood%2520reference.doc&usg=AFQjCNELQkm9WM1xYL-ut4qkvMRgHHG7Hw&cad=rja
http://www.strath.ac.uk/careers/uni_staff/writingacademicreferences/exampleofacademicreference/

Edinburgh has a useful guide with questions it thinks are relevant, that cna form the basis for a reference.
http://www.ed.ac.uk/files/imports/fileManager/reference-request-template.pdf

Cambridge gives a very detailed guide, which might give you an idea, but this might be overkill.
https://www.mrc-cbu.cam.ac.uk/documents/equality/HHMI_WriteReference.pdf

Be aware there are many reference guides for US unis, which may have a different style.

Theres enough there to get you going.