Here's Hexuba in a newly-created lair for her
Once you see the Green Ranger in his nightmare then that's new footage but once you see this shot then it's the start of Sentai footage.
As you can by this shot, the shot of Damon's Jet Jammer has been edited into Sentai footage but once you see this shot of the Green Ranger riding on his Jet Jammer then it's back to new footage.
Once you see the Blue Ranger in Kai's nightmare then that's another start of new footage.
Here's new footage of Swabbies
This shot of the Green Ranger in midair is Sentai footage which comes from Episode 18.
Footage of the Blue Ranger fighting Swabbies while he's in the water is also from Episode 18..
Once you see the 2 female Rangers in the Nightmare world then that's also another instance of the beginning of new footage.
Once it cuts back to the Green Ranger is when it goes back to Sentai footage as seen in these following shots:
 

comes from Episode 37.
is indeed Sentai footage. Once it cuts back to the Blue Ranger then it's back to new footage but once you see this shot of the Blue Ranger and a Swabbie is when it goes back to Sentai footage. This shot of a Swabbie and the Pink Ranger is when the Nightmare World goes back into new footage.
This shot of a bunch of Swabbies is also new footage.
Once you see the Red Ranger in the Nightmare World then that's another instance of the beginning of new footage in the Nightmare World.
Once Mike says as he's walking through Terra Venture "gotta keep moving," it then cuts to Sentai Episode 8 footage of the Swabbies as seen here however it then cuts to new footage of the Green Ranger.
Here's a new shot of the Pink Ranger with her Beta Bow
Here's a Swabbie sparking up in the Nightmare World as a result of Karone's Beta Arrow attack
Once you see the Nightmare Monster then that's another start of Sentai footage.
These shots of Nightmare punching Red
is when it goes back to new footage. This specific shot had to be reshot since in Gingaman, Nightmare tries going for Red's helmet.
These shots of the Red Ranger
  is when it goes back to Sentai footage.
Once Mike breaks Hexuba's Crystal Ball, it then cuts back into Sentai footage of the Nightmare World battle between the Nightmare Monster and the Blue Ranger.
This shot of Nightmare is when it goes back to new footage, it then cuts back to Sentai footage once you see the Green Ranger high in the air.
Nightmare Monster grows automatically. His growth sequence is all Sentai. In Gingaman however he drinks a Potion before he grows.
After the Megazord destroys the Nightmare Monster, it then cuts to Sentai footage of Hexuba in her Room.

All snow footage looks to be new footage.
Sentai footage comes from Ginga Episode 46