Outlining an region of interest to count in TIFF

To someone new to Photoshop you may find the pen tool a bit of a challenge. Perhaps another approach would serve your purpose. It takes longer to describe than to do.

Add a blank layer and use Cmd+ to enlarge the view.

Choose the Lasso tool with the Feather in the Options bar set to 0, to draw the outline. (I'll admit that drawing with the Lasso is a bit like drawing with a bar of soap, but don't  be put off by the lack of accuracy at this point.)

When you have made the selection, click on the Quick Mask tool (it is below the color swatches in the Tools panel on the left of the screen).

Use a hard edge brush set to black black to add to he red area. To subtract choose white (to erase red).

When you are satisfied with the correction, click the Quick Mask again to return to the marching ants.

If you discover a point in the selection you want to adjust, just click on Quick Mask again to make the change, and then return to the marching ants. 

When you are satisfied with your selection, choose Edit > Stroke and set the width (perhaps 2 px), the color and Outside. OK.

Then Select > Deselect to eliminate the marching ants.

You can now drag that layer in the Layers panel over another image and use the Move tool to position it. With the Move tool selected you can also use the arrow keys on your keyboard to move your outline in one pixel increments.