Trainer Road is a good start. But if you want a REALLY cheap option, just use any speedometer (on the back wheel). For a turbo, speed is power anyway.
Then it’s interval time! Pyramids of 1-2-3-4-3-2-1 minutes as hard as you can, with the same recovery between. Then for another night, 2×20 min very hard. Finally 20/40 sprints – 20 seconds full on, 40 seconds recovery. 10 of these will finish you off.
Don’t forget you need a 15 minute warm up and a 10 minute cool down.
FTP is all well and good, but speed is just as effective if you use the same bike on the same turbo.
And check your tyre pressure each time for consistency.
I’m personally think Sufferfest videos, or youtube training videos are more useful than Trainer Road. But anything that actually makes you do the sessions is a bonus.
Oh and I do mine on the road 😉