For the most part, you will follow the same steps as if you were to own and house the VoIP system yourself. You still need an expert on your side. You still need to design your call routing and you still need to select your phones. In this case, however, the selection of the vendor takes on a different perspective. This vendor is your installation team, ongoing support and maintenance plan.
Besides feature sets, hosted VoIP vendors need to meet several other criteria. Financial stability should lead the list again, along with average downtime, responsiveness, customer support, administration, ability to support multiple sites, termination, security, etc. All of these should be outlined in a Service Level Agreement or SLA.
Up-front costs are much lower for a hosted solution when compared to owning, although you will still need to purchase the phones themselves. A recurring monthly fee is assessed, which is typically based on a per-user basis. This can can have an effect on cashflow over the long term.