My method:
Get a piece of paper and fold it down the middle. Pop out the tablets on the piece of paper and use the bottom of a glass to crush them one at a time. When you're done, pick up the paper and use the fold to help you tip them in the glass.
Add lukewarm water, stir till dissolved and put in freezer for 5-10 mins, till the water is cold (not frozen or forming ice). Get an old pair of boardshorts and wet them thoroughly. Get another glass and lay some of the boardies material over the top of the glass (only use one layer of material). Poke it in a bit to form a 'pocket' then hold this tight around the rim of the glass.
Stir your solution again and pour it into this little pocket until it's almost full. Carefully squeeze this pocket so the water goes into the glass below. Tip out the strained solids, repeat as necessary till you've strained all the liquid.
I find boardshort material works really well cos you can squeeze it really tight and it won't break like paper filters, plus the holes are quite small so nothing but water will get through.
On how much water to use, some people say only 50ml, but if you lose 5ml of water, there's 10% of your codeine gone. I use about 10ml/tablet, so 24 tablets use about 250ml. That way if you lose 5ml, you're only losing 2% of the codeine.
I don't think brand makes any difference, I get Panafen Plus (same ingredients as Nurofen Plus) cos it's cheaper. Should only take a minute or two to dissolve if you've crushed the tablets. Entire process including cooling should take no more than 15 mins.
Well it sounds like your freezing the codeine+water solution. It should be a COLD water extraction not a freezing water extraction. Heck, you shouldn't even worry about the temp too much. Try just cold tap water with no refrigeration and be more mindfully of how it filters. Filtering is far more important than the cooling.
The whole reason this process works is because of the cooling. The codeine dissolves in cool water but the ibuprofen doesn't. So you can use lukewarm water to dissolve, then cool the mixture so the ibuprofen won't be dissolved, OR use cool water from the start and only the codeine will dissolve. Personally I use lukewarm then cool, only because it's easier to dissolve the tablets in!