I just started using a system this week I made from reading up on this topic on the internet.
I made a simple "CAR CARD" for each piece of rolling stock. The data for each car is:
ROAD NAME (Seaboard Air Line, Wabash)
CAR NUMBER (20202, 39465)
CAR TYPE (Boxcar, Tankcar, Gondola)
and a WHEN EMPTY RETURN TO line (like a yard or somewhere offline)
Below all the data the card is folded over and taped to create a pocket to hold the Waybill.
Next I made some simple "WAYBILLS" with the following information:
TO (which industry on my layout gets the car)
CONTENTS (oil, wood, logs, TVs, appliances, sand, coal, even EMPTY)
CAR TYPE (need to know what kind of car is required to transport the commodity)
The Waybills (with the printed data on them) have blank lines next to each field so I can write in the industry, contents, and car type as I start the operating session.
Both were made using MS Excel, I used card stock (available at Walmart) to print the car cards with and plain printing paper for the waybills. Once I got one like I wanted it I copied/pasted it so I would have 9 on a single sheet of paper to save paper when I cut them apart.
With the Car Cards made and a few Waybills printed up I then decided how many cars I needed to deliver by rolling a 6-sided die (1-6 cars max is about all my layout can handle at one time) but two 6-sided dice will give you 2-12 cars.
I then fill in the Waybill blank lines with the required data and place the cars on the "Arrival" track so I can make up my train.
When I set out each car I place the Car Card with Waybill next to the car. If the Waybill says EMPTY that means the car was delivered for loading. Cars like that then need another Waybill (I use a single entry at at time, some Waybills have 4), perhaps for the next operating session.
Next operating session I first look at the cars already on the layout and decide if they've been emptied or loaded and pull the Waybill from the Car Card or make a new Waybill to move the car to its next destination. When the Car Card has no Waybill it goes back to the yard (and off the layout).
As I operate the layout I'll print more Waybills for future use. The Waybills and Car Cards I've used I place in the "holding box" behind the ones I haven't yet used. That way I'll be sure to use all my rolling stock and eventually I'll have enough Waybills to cover just about every commodity my industries will make or use.
One goal is to pickup a car at each siding where I drop a car so they won't get full. If I find one that can take no more cars I can always fill out a Waybill to pick up empties or cars that have been loaded with stuff for "Offline" use.
I'll takes photos of my Car Card and Waybill and post them later.