Start with the CPU utilization for the SQL Servers. Go back over a 30 day period, and take note of daily processing cycles. Check month and year-end requirements. My experience has been that there will be a few servers that are maxing out, and a bunch that barely do anything. The latter are good candidates for consolidation.
Group the servers by business owner, and take note of SLA requirements. Try to come up with an SLA matrix. It may be technically feasible to consolidate onto two servers, but not if one or more applications has a maintenance window totally out of sync with the rest.
Get this information to the right person as early in the process as possible. It will be a lot easier to get the funds for an extra server or two that way, if required.
Review your license inventory. Depending on how you are currently licensed, you may (or may not) find it advataadvantageousvor one solution over another.
Here is an article from Microsoft on the subject: