Make a duplicate layer of the background layer, call it <working>. You will work on this layer while leaving the background layer intact.
Make a new layer. Select All and Fill with white. Move this layer just under the <working> layer.
Using hand-selection techniques (any lasso tool or pen tool with conversion to selection) you will be dividing your film still up into 30-40 divisions. Each division should be, as much as possible, one value or a close set of values. Be sure that no feathering is turned on in the selection tool's options - so that you get a hard-edged selection.
In this assignment, you want to reveal the tonal structure of the image. Choose your divisions (or forms) so they each contain only very similar tonal values; if not, important values will be lost when you average for value.
Reminder: You must use hand-selection techniques, not magic wand or color range, for this project. The difference is easily visible.
Make your first selection on the working layer. Do Edit/Cut. You will see white through the gap. Now do Paste-Special/Paste in Place. This will paste the cut portion onto a new layer, exactly in the place it came from. Name the new layer <layer 01>.
Select the form only (and not all of the transparent area of the layer as well) by Command-clicking in the layer's icon - on the small rectangle. The form will get a selection line around it. Use Filter/Blur/Average. This will turn the entire selection one value. Decide if the averaged value represents your selection well. If it's too light or too dark, you might want to revise the selection.
Note: If you make the mistake of averaging the whole layer, the value will be far too light.
Note: Do not change values manually. They must be set by using Blur/Average.
SAVE, and then turn off the new layer's visibility so that you can see the gap in the <working> layer.
Make the <working> layer active again. (Hint: you can Command-click on something in that layer to reactivate it)
Make your second selection. Do Edit/Cut and then Paste-Special/Paste in Place. This will paste the cut portion onto another new layer. Name the layer <layer 02>.
Blur/Average the second form, as you did with the first, and decide that it's a good tonal representation of your selection.
SAVE, and then turn off the new layer's visbility so that you can see the gaps in the working layer.
Keep doing this until the entire working layer has been entirely cut away. SAVE frequently. Keep numbering each new layer that you create. Remember that forty selections is your maximum. If you find that you're going over forty, you'll have to combine some layers and re-average them.
An example of a film-still only partially converted into averaged-value forms. (This is incomplete.)