Don't. Just write what you're trying to say. It rarely works well to write in slang. A few authors have mastered the technique, Terry Pratchett for example. Most of the time though it just looks stupid.
'Aye, I'm wa hame th'noo lass', Donald said.
'Aye, I'm away home now lass," Donald said in his thick Dundonian accent.
The first one just stops readers in their tracks, trying to decipher it. The second one gives you the idea of a scottish voice by word choice and attribution hints. The first may be more authentic but the second keeps them reading. Same goes for slang. If it isn't generally known, don't use it.
(Incidentally, it's not slang you're using, it's accents - slang would be 'up the apples and pears' for stairs in cockney, or mullered for drunk.)