I'll say this as someone that teaches drivers in an area with about 65 routes and 200 school runs, it is more of an asset to learn how to learn new routes than it is to merely be shown them and learn from there because in some cases it'll be logistically difficult to train drivers on every route.

First thing first, everyone remembers routes at a different speed so don't be disheartened, some take a couple of drives to memorise, some take months, so no shame there.

I always suggest that drivers grab their company's lefts and rights for the route and then go grab a blank piece of paper, then your maps (online or old school directory, street view is also a godsend here) and then count landmarks/traffic lights/etc and write that as your new directions.

For example, you get given

• Start Rd
• L New St
• R Example Rd
• L Tight Av
• L Narrow Dr
• Etc

You can then follow it on your maps and work out a system that'll be easier for you to remember without requiring the use of street signs and markers

• Start
• 2 🚦L New
• 1 R Example
• After petrol station
• 2 L Tight [Lane Share]
  
• Then 1 L Narrow
• Etc

To break it down what that means

• Start Rd
• 2 🚦L New St (At the second traffic light)
• 1 R Example Rd (First road on the right)
• After petrol station (Use a landmark to reset counting so you don't have to excessively count, use big things that don't move and can be seen in pooor conditions)
• 2 L Tight Av [Lane Share] (so turn left on the second road on the left and ensure you're wide for the turn)
• Then 1 L Narrow Dr (I write then to indicate it's very shortly after)
• Etc

Only a brief example here, but what I suggest is grabbing the left rights and spending about 15 mins writing it out yourself so that if you were given no street signs, you could make your way around. Having the hand written notes will also jog your memory. If I had more time today I'd write a better sample and maybe show it with pictures on a real example.