Here's a complicated unit - ten Chaos Marines: six normal, two meltaguns, icon, champion, so four wound allocation groups.
Here's the unit taking ten wounds of the same type. Each group is full up with one group of wounds. Six on the bolter guys, two on the meltaguns, one on icon, one on champion.
If we go by the dice results here and say they're normal wounds they get saves against, two bolter guys and one meltagunner will die.
Now we mix it up a bit. The red dice are still something normal, like lasgun wounds, but now we add the black dice as for example plasma wounds, which will ignore the CSM's armor save. Four plasma wounds and ten lasgun wounds.
All four plasma get allocated to the regular joes, along with two lasgun wounds to fill them out, the meltagunners, icon, and champion each get a lasgun, and with two lasgun wounds left over, they lap around back to the bolter joes.
If we go by the dice as shown, four bolter guys will die (no save) and one meltagunner also dies from a failed lasgun save. If you ignored wound allocation in this instance and just did everyone as one large group, you would only have five bolter guys die and the meltagunner would be safe. Using the allocation properly provides an example of how wound allocation can benefit the shooter, even with mixed AP weapons.
Here's another mixed AP example, this time with the white dice representing the armor-ignoring attacks, and red dice still being normal armor saves. Say, eight Demolisher cannon wounds, six heavy bolter wounds.
Six demolisher wounds go on the bolter carriers, and everyone else claims a heavy bolter wound until everyone in the unit has a wound allocated. Then in the second lap, all of the remaining wounds (two demolisher and two HB) wind up on the bolter carriers.
With the dice in this example, the bolter guys are wiped out with a little left over, while the other groups are safe. This is one of the examples those who dislike WA hate; if you just did everything as one group, you'd wind up with nine dead CSM (eight from no-save demolisher, one from failed HB save). Interestingly though, the CSM player would get to choose which one survived.
Let's make the squad smaller. Three bolter guys, meltagun, champion - three wound groups.
Using the dice as shown, everyone is OK except for the champion, who bites it. Without wound allocation, rolling the six saves as one group, you'd just see a regular bolter guy go down instead. Another example where WA can benefit the shooter.
Two plasma and a lasgun go on the bolter guys, lasgun to meltaguy, lasgun to champion. Three lasgun wounds left over, wrap back around to the bolter guys.
Here, dice as shown would have two bolter guys die (no save) and the champion bites it again. Without wound allocation, you'd just see the three normal bolter guys die instead.
One of the much less likely, but commonly used examples - a bunch of normal saves and a bunch of amor-ignoring ones. Again, white dice for armor-ignoring shots, like plasma, red for normal shots like lasguns.
Three plasma to the bolter guys, lasgun to melta, lasgun to champ. Wrap around - repeat three plasma to bolters, lasgun to melta, lasgun to champ. Two lasgun wounds left wrap around a second time and land on the bolter guys.
This is an example those who dislike WA really hate, as it only has the three bolter guys die, where if you ignored allocation the squad would be wiped out by the plasma wounds.
For the last example, let's tie it in with another of the big 5th-edition hates, cover saves. Here the five-man CSM squad is in a trench, hunkered down for a 4+ cover save. They're hit as in the last example, with six amor-ignoring plasma wounds and six lasgun wounds, but this time they get to make cover saves against the plasma.
Allocation as previously, but this time the CSM get lucky with their cover saves and unlucky with some lasguns, felling both the champion and the meltagunner but sparing the normal joes. Without wound allocation, rolling as one big group, you'd see two bolter guys removed instead.
So there's how to do it. Keep track of how many and what kind of wounds your opponent inflicts, grab some dice, put them in groups, roll, remove casualties. Try it a few times, it really doesn't take long. If you try the above scenarios (or others) and roll them out a few times, you'll see that sometimes wound allocation ends up benefiting the attacker, sometimes the defender.
You may or may not like it, and that's your right, but I hope that you won't consider it "too hard".