Best tool is : NetApp 7-Mode Transition Tool 3.3.1 Copy-Based (7MTT), it supports Souce Storage OS : 8.2 and Destination Storage OS 9.3. So, it fits your requirement.
Regarding the steps, there is plenty of notes around this on our community forum, probably too much of information already discussed. So, I would suggest just search it on our forum or simply google it.
However, when you download the tool, you can also download all the necessary documentation, such as :
Product Documentation a)Installation and Setup Guide b)Copy-Based Transition Guide c)SAN Host Transition and Remediation Guide d)Release Notes
This is good enough to get you started. Inumerable customers have used this tool for migration purposes and it works great, just that some planing is needed from your end. Of course, it goes without saying that you must do all the Interoperability checks on all the components that will be connecting to your 9.3 ONTAP, so that there are no surprises or dissappointments!