Start by crushing up the pills and weighing the powder, if you have a scale. If you don't have a scale then keep the box with the total amount of APAP and dhc. That's a lot of pills and powder so you're probably going to have to use 200+ml of water, you can eyeball it based on the amount of power but just be sure to measure the amount of water you're using, either in ml or fl.oz. With this much dhc im assuming you aren't taking one dose, which makes the measuring water part very important. Use as little water as possible because the more water you have the more APAP will be in your final solution.
After you measure the water you can go ahead and add your crushed pills to the water, stir the solution thoroughly for a few minutes but don't add any heat. At this point the dhc and some but not all APAP should be dissolved into the solution. Then place your solution in the fridge and let it sit in there until it's as cold as it will get, be sure to open the fridge and give it a quick stir at least a couple times.
Before you pull the solution out of the fridge pre wet two coffee filters and affix one of them to a suitable container using a rubber band or some improvised clamp. Pull your solution out of the fridge, you don't have to be lightning fast but don't let it warm back up. Give it one more quick stir and start carefully pouring the solution through the filter into your container. It's going to be pretty slow to filter so don't pour it all at once. Switch to the other filter if the first one fills up with junk.
Then when you're done filtering you should have a pile of APAP loaded gunk in the filters and a slightly cloudy dhc solution in the container. You can then portion out your doses by dividing the total amount of dhc in the pills by the quantity of water you measured. Your dihydrocodeine is in the water, everything left in the filter can be trashed (or set aside if you want to be extra anal).
If you absolutely want to make sure you did it right then take the weight of the pill powder you got at the beginning (if you used a scale) and compare it to the weight of the completely oven dried APAP gunk. Then you can make some rough calculations to see if you got most of the APAP out.
Sorry if this is long winded but I wanted to be thorough since you are dealing with enough APAP to potentially kill five people.