I ramble a lot in this one, so here's a simpler TL;DR version for you:

How To Make Your Scanned Artwork Transparent in Photoshop Using Channels

1. Paste your Lineart into your Comic Template, or make sure your lineart layer is actually editable (double click on it, then hit enter)
2. Increase Contrast Using LEVELS (Image > Adjustments > Levels or Cmd/Ctrl + L) to eliminate any grey haze or debris/clarify your linework/increase the amount of white
3. Turn off the visibility of every layer except the scanned artwork in the Layers Tab
4. Open Channels Tab (Next to the Layer Tab - note: Channels are not available in PS Elements)
5. Cmd/Ctrl + Click on the Channel THUMBNAIL
6. Return to Layer Tab
7. Make sure your Scanned Artwork Layer is the one selected
8. Hit the DELETE button
9. Deselect (Select > Deselect or Cmd/Ctrl +D)
10. Lock the Transparency of the Layer
11. SHIFT + F5, Select BLACK from the dropdown menu, then hit ENTER/OK

ya done son

There are programs that offer much easier ways of doing this that I need to look into/learn, but it might be helpful for you fellas who are more comfortable working in Photoshop.