I have self employed expenses (ex. chair & desk) but, I also an employee and would like to claim work from home expenses (T2200). How to I do this?

You would have to figure out how much you use your home office for employment, and how much you use it for self-employment, as well as any personal use. Then you need to enter your expenses in each section based on usage. Ex: Your electricity bill is $1000 for the year. You used it 50% for employment, so you would enter $500 as a home office expense. You used it 25% for self employment, so you would enter $250 in the self-employment home office expense section. 

The desk and chair you likely won't be able to claim as an employment expense, but can claim the self-employment portion as CCA (Class 8).