What steps should I take to prepare for using a thermal camera?
Started as a comment but became two long;
No privacy problems you don’t have to join to use the FLIR one that is the main objection I am aware of and I have not joined.
I have the pro model and to tell the truth I would do nothing different , you want a constant temp for an extended time if changing temps that can create shadows in concealed spaces that may show issues if a constant temp. You can find leaks in the ductwork but the system needs to be running in some cases to identify leakage from duct works, running the hot / cold water for a couple can help identify bad insulation on your pipes. You may also identify roof leaks in the attic , water running down is cooler and I have found some I could not see. Another thing you can do is look at your breaker panel and outlets. A loose connection can really heat things up ( I use my unit at work to find electrical problems and bearings that are running a little hotter than normal) , so you do want the forced air system running small leaks spear as pin holes or long straight lines at leaking ducts. 
Rain will cool the roof and make leakage show up plus as I mentioned leaks you may not have found may be spotted.
I have used FLIR cameras for quite a few years , I no longer pay for an annual evaluation at my plant saving $ and I find motor problems prior to failure in some cases.