Understand Business Model
Garbage in is garbage out, so before going into numbers you need to make sure you fully understand all the implications of your business model.
Challenge Costs & Revenues
Assess every block of you business model canvas and challenge yourself what kind of impact this has on your costs and revenues. When you have covered this, you are ready to build you financial model.
Validate Financial Model
Just as you validate assumptions about your customers, you need to validat eyour financial model. You do this by asking an optimist, a pessimist and a realist. Together they will give you the balanced feedback you are looking for.