I find it easier to focus on what is not fun and then do the reverse. Some unfun things in maps include corridoors, flat lighting (no interesting shadows), anything frustrating (taking falldamage right after spawn, or mazelike layout), crazy long sightlines and what have you.
Balance <=> fun. If the map is fun to play for all classes then it usually is balanced. I tend to view my maps from the perspectives of a sniper, engineer, scout, soldier and spy (also somethimes demo), and try to find ways to "break the fun" or grief the other players. For instance a long sightline to spawn -> sniper camp heaven. A blind corner at a choke -> pyro/spy/engi heaven. A window from where you can safely stickyspam everyone to death -> demo heaven. I could go on but I guess you get the picture. Every powerful spot on the map must have some draw back. An easy fix is to place the item/health packs somewhere else, but I digress.
Note these things above apply for any map (in any game really). What sets payload maps aside from CP/KotH/Arena is that the objective is moving. This really doesn't amount to a whole lot of differences what I've found, as there are still points to capture in PL. One nice rule to follow is to have the payload on the lowground, so that the players pushing it are more exposed/cannot hide behind the cart so easily. Also, medium+ health packs are usually placed some ways away from the cart, so that the pushing players must retreat if they get hurt.
I bet you can find a more complete PL tutorial somewhere on this very forum.
If you are new to the mapping scene, one good thing to get right from the beginning is scale. Both too open/tight maps cause problems. How I dealt with this when beginning was to use the sentry radius to visualize distances in official maps. I then checked that my maps had about the same distances. I also counted seconds from wall to wall for longer portions.
TL;DR Start by making a working map and iterate the unfun things away from it.
A variety of routes for Blu to use to attack Red, each with benefits and drawbacks, makes a map fun. On the flipside, don't make it too hard for Red to defend. Consider sentry spots, and balance them to prevent them from being overpowered, without making Engineer useless. Also note where Engies will place their other buildings. Consider where Red players will congregate to defend the cart from as it inches closer to the point. Upward points A and C are great examples.