The hardest part is getting your risk tolerance assessed.  Once that is done, the most difficult part of the process is to determine the equity/fixed income ratio mix.   ETFs can cover the Equity (Canada, US, International and Emerging Markets) and Fixed Income (Cash, GIC, Bond etc) portions.  About six ETFs could cover it all off.  Rebalancing involves putting new money towards the worst performing asset while keeping your initial ratio mix in mind.

Simplicity, diversification and low cost.