How do you write a Scottish accent?

Basically, unless you yourself are Scottish, try not to lay it on too thick. Keep most Words simple and as close to English as you can, but change the dialect. It's hard to explain.

Example: I didn't know she was so pretty.

With an accent: I dinnae ken how bonny the lass was.

Easy to read, but you still heard an accent in your head didn't you? There are a few resources available on the internet, the most popular one for English to Scottish dialect I've seen is in the link below

Remember, you're trying to make this character a believable person, not a caricature.