Grab the magnifying glass tool and zoom in on the center of the image, preferably in a spot that has a lot of detail. We're going to want to go up to 2-300%. Yes that takes a lot of screen real estate, but that's why they have scrollbars.

Here's a good spot

Now in the channels window, make the blue channel active. If all goes well that's all you'll see in the editing window in all its grayscale glory, and everything but blue will be unhighlighted in the channels window. But we need a reference, so turn on the eyeball icon (eyeball only, don't highlight it) next to red (contrasts well with either green or blue, which is why it makes a good target for this). (You might want to do a save here as well, and close the tiff window if you haven't already done so, we're finished with it.)

Using the Move tool, line it up as best as possible manually, and then do the final fine adjustments with the arrow keys. It's helpful to turn the eyeball icons in the channels window off and on to see if the image jumps up/down or left/right a pixel or two, then it'll need to be nudged some more. Flip back and forth until there's no movement.

Then edit the green channel, turn off the eyeball for blue since you're done with that. Leave red on and we won't have to move that because that's our target. Repeat alignment procedures for green, and then turn on RGB to see what you've got.

Should look something like this

Now grab the Magnifying Glass tool again, back up and see what you've got. Might do a save also.