Now while looking back to the process I followed for the upgrade, I can say that the first thing to do is to identify the approach to follow. Plain and simple, there are two approaches. In-place upgrade, fresh instance upgrade. There are many posts that talk about the advantages and disadvantages of each approach (I won't go into those details here). Please read about it and take an educated decision on which one to follow based on your project.
When I was going to make my decision, an architect told me that it depends on the repository, code, integrations, time. A lot of times, doing a small POC of the approach that will be taken is the best way to identify which one is the right one for your solution.
This is how I ended up making the decision to go with an In-Place upgrade, it suited the solution needs plus it is the recommended approach by Adobe.
Once I had the approach, I had the 6.5 jar file/license, I was ready to build the team to perform it.
Upgrades take time, although it is not at all a project where you can put several engineers to make it go faster, upgrades are 1-2 men job at most.
Now that we have identified the approach, that we have the files ready for us, and that we have the team that will do it, is time for the planning around it.