There is a lot of difference between inexperience and laziness. 'People like me' have a great deal of patience for inexperience. We also are often the inexperienced ourselves as no one person is expert at everything. Laziness is a different animal.
Since you have put in some effort, I will fill in some blanks for you. You have already figured out that you do not actually 'convert' 24V to 5V, so no need to cover that.
You do not need a diode unless you are driving a load that will create a surge that needs to be dumped. Mechanical relays have coils which generate surge when they are shut off and the field collapses.
You will encounter diagrams that show a device identified only as a 'relay' with a diode installed around it. This is typically not necessary with solid state devices.
The IMPOTANT current spec to design to is the MAX load on the DRIVE I/O and to be aware that some drives list max PER pin and some list a TOTAL for all pins. Obviously, do not exceed either. At 20ma, undoubtedly you are below the max on your drive, but be aware of the total if that is how your drive is rated. The LTV can handle enough current to fry most drives, so just limiting enough to protect the chip is not enough. Your choice is very conservative so you are good to go.
I use a 400 ohm power resistor on my Mitsu interfaces. You may want to consider the type of resistor you are using as it will probably get very hot. Power resistors can handle the current indefinitely, provided you space them up a bit off the PC board.
Take the 5V + and the 5V ground off the BOB. Do not use the PC power supply or any floating source for the 5V signal source.
Check the BOB inputs for the presence of pull-up or pull-down resistors and choose your active high or active low appropriately. This info will be in the documentation that came with your BOB. Often you can choose the state with jumpers. You can wire the 5V side of the LTV for either.
If that last statement is not clear, just call 1-800-KnobHead and I'll see if anyone can help you . . .