The “quick method” for calculating your GST liability relies on your revenue amount only (ie does not use input tax credits from the GST you spend).
You must be registered to file using the “quick method” with CRA. By default, you would be a “regular” filer and would need to explicitly register to use the “quick method”. You can change your method, but must contact CRA to initiate this change.
Your decision may depend on your business’s GST “ins and outs”. If you have relatively low expenses, it may not be worth tracking the GST on the items as they would result in low input tax credits (ITCs). For example, if you have a consulting business with very little expenses, it may make sense to use the “quick method”. However, if you have a business with larger numbers of expenses that incur GST, it may be more lucrative to remain a regular method filer.
This document outlines the calculation method for the periods before and after the HST and the period when HST was in place.