I am soon going to embark on the SS1 training plan which will be my first time ever following a structured training plan. However, before I start I do have a few questions regarding power tracking. I do not have a power meter on my bike but I do have access to an indoor trainer which can track my power. I was planning on using this to perform my FTP test but I am not sure what to do for my interval/outdoor training. I would obviously love to do as much as I can outdoors. What should I do? Would it be ok to train intervals outside without a power meter? Should I do some intervals outside and some inside? If I can do them outside what can I do to not overcook the intervals and do them correctly? What metric would be the most useful to watch when riding outside?
I was in a similar position, smart trainer indoors but no powermeter on the bike. Ultimately the plan made me realize that I wanted to get a pm on the bike also to maximize my training, so you may end up getting to that point also.
I did an indoor FTP test, that let me set the targets for indoor workouts and get a feel for the intensity, and then for SS1 most (maybe all?) of the efforts are steady state >5mins, so heart rate or even rate of perceived exertion (how it feels) give you a pretty good idea of how you are going. Or at least that’s what I did and I definitely got faster.
But now at the end of SS3 with more densely structured intervals and wanting to do them outside I decided I need a powermeter.
Either way: power or heart rate or by rate of perceived exertion (RPE) you can follow the plan - that nets you your greatest improvement 
For powermeters you can use your FasCat Team Discount at our partner ExcelSports - they sell, Stages, and Quarq powermeters which are two of the best. You can get a left crank Stages for $350:
https://www.excelsports.com/main.asp?page=7&major=3&minor=20