This is assuming you are expected by your council to clean it “thoroughly enough”.

After you eat/drink the contents, a very quick rinse is all that’s needed. Little stains aren’t going to be bad for recycling.

If you’ve left it for a while and are worried, a simple but surefire way to clean out any significant gunk would be these steps:

• Make sure you have a strainer on your sink drain if your plumbing system is fragile.
• Fill every can or jar with water to near the top
• Wait 15 minutes
• pour out a can into the sink
• fill the can with a bit of water and shake the last bits out
• pour and lightly rinse the rest of the cans in the same way
• bin the gunk caught by the strainer.