"..the manuals aren't usually that clear in their descriptions, but if you look at the photo that they usually have, by looking at which side the trailing arm is visible in the photo and make sure the shoes are fitted that way on that side of the vehicle ..."

The "gap" ie the bit of shoe without a lining is the leading edge of the shoe. So, if you can't remember which way 'round they go (like me) just imagine the drum turning with the car going forward. The part of the shoe that the drum contacts first as it rotates is the bit with the cutout.

Another trick when adjusting shoes is to make sure the shoe is concentric with the drum. Spin the drum, grab the cable and give it a tug. This will brake the drum and move the shoe so it is concentric. Now give it a final adjust - it may not need it but check...

One final trick.. if the trailling arms have been off, the cable quadrants may have been installed upside down. They are not symmetrical and, if if fitted wrongly, will jam against the trailing arm (fully tensioning the cable) before the shoes is fully clamped against the drum.

Good luck, Ian