I bought a large peice of equipment for my business How do I write it off?

You can't "write it off". You need to take CCA on it. You would enter it as an addition. You can find the class on the CRA's website: https://www.canada.ca/en/revenue-agency/services/tax/businesses/topics/sole-proprietorships-partnerships/report-business-income-expenses/claiming-capital-cost-allowance/classes.html