As for how to MAKE an overlay, I've developed a foolproof process. You first need Google Earth Pro. It's free, and you can
download it here. When you install it, use use your email address and the key GEPFREE to sign in. Once you have it...
1. Find the area you want to make the overlay of. I recommend always making your overlays North-up (this way you'll be building the coaster in the correct orientation), so click the N on the compass in the upper right corner to do this.
2. Go to File>Save>Save Image. A toolbar will appear at the top of the screen. Change the resolution to Maximum. This is why you're using Google Earth Pro, the regular Google Earth is limited to your screen resolution.
3. Resize the window until the resolution shows 4800x4800, which will give you the largest and highest-resolution overlay possible.
4. Without resizing the window again, zoom and pan the view until it's exactly how you want it. Turn off any map labels you don't want on the overlay.
5. This is the most important part. You need the image scaled accurately in NL2. To do this, in Google Earth Pro go to Tools>Ruler. Set it to Feet if your NL2 is set to Imperial units or Meters if it's set to Metric. Click on the absolute far left of the view window and then the absolute far right, making sure the yellow line is completely horizontal. Write down the exact Map Length figure. Once you've done this, do not zoom, pan, or lose the number, or you'll have to re-do it.
6. Click the Save Image button at the top and save it. As long as you have the Map Length number, you can close Google Earth Pro now.
7. Open the NL2 editor. Go to File>Preferences>Overlay tab. Open the image you just saved.
8. Here's the whole trick. Use a calculator to divide the exact Map Length number by 4800 (not the other way around!) and put the result in under Scale. Don't round it or anything. This will make the image
exactly its true size, and if you're doing this for a single coaster the overlay will be
ridiculously high-resolution.
9. Save the overlay setup and click OK. If you need to, enable the overlay display in the top viewport.
This is tried and true to give you the best possible overlays, all the ones in the original post including the one for the whole park were made by this method.