Few newbie questions

Hi!

I have been doing the linear routine for 3 weeks now and I have a few questions.

How do I know I’m doing it good?
What is nocturnal wood? (I know what morning is, but never heard of noctrurnal, but people keep mentioning it here)
Is it good to massage it after or before the workout? I seen some people here say it is.
When do I start to do 2 days on and 1 off?
How important is warming down after workout?
Can you please tell me which stretches are the best and safest?

Thanks! :)