Letter to a new filmmaker after shooting his 1st roll...
Congratulations! You are now a filmmaker!
WHAT IS THE 20 SECONDS OF BLACK?
The 20 seconds of black is a leader (unused film) that is wound onto the reel before shooting. There will always be some 'lost' film as 'black' from winding onto the reels. This is typical. Some developers may add a white colored leader to the beginning of the finished film.
This allows threading of the film through the projector from the supply reel to the take-up reel without using any 'good' film footage. In your case, the developer has glued both ends of the two pieces of double run film together after processing.
FLATBED OR EDITOR
What you would do next is to edit the film on a "flatbed or editor" to remove the black areas of film and to edit for content and then add a white leader to the beginning. Leader material:
The editor or flatbed looks like this:
Typically one would run the film up to the black portions and cut out the black to make an seamless edit from one section to the next. The editor usually has a device to hold the film so it can be cut with a single-edge razor blade.
After cutting the two ends to be joined are roughened up a bit, overlapped one frame, and then cemented together using film cement:
HOW TO SPLICE
Here's a PDF on how its done. The PDF talks about Super 8, but all film is essentially done in the same fashion.
The PDF also speaks of "Kodak Presstapes". These are double sided tape used in place of film to splice two pieces of film together. Cementing the two pieces is also common. Some people prefer splices; some prefer glue. I like glue, as I learned it that way. if you decide to do your own editing, practice first on some old footage. You can find exposed reels of film on eBay.
As the footage with be 3 minutes or less, you can mark the box the reel came in with the subject and date, etc. You may want to edit and the collect like footage for inclusion on a larger reel. 7" reels are common and fit most home 8mm projectors.
You can find the reels on eBay. Make a printed sheet of title indicating which subjects are on that particular reel and tape it to or put it inside the reels box. You should also gives dates, times, places, etc. 20 years down the road you be glad you did :)
Re: Exposure. I recommend keeping a log of f-stops settings were used. And then afterwards, some notes saying what worked, and what didn't. Too light? Too dark, etc. You'll get a feel for how the film works and what adjustment to make when shooting.
Be sure the projector bulb is the correct size for the projector. Some people will use a smaller (or larger) bulb. Start with the correct wattage bulb for you projector. Since you'll be showing footage you shot (compensating the light with the cameras f-stop settings), both what you shoot and what you project are interactive.
Also note that some 8mm projectors have a light control. Sometimes its a 2 position switch and sometimes it's a knob. With it, you can increase the brightness of the projected footage.
TRANSFER TO VIDEO
Finally, you can have the film transferred to video. Transfer houses can adjust light or dark footage during transfer. The adjustment will be small. Typically they cannot correct footage shot several stops too dark or light.