I am always trying to do stuff on a dime and I am the least mechanically skilled man I know....but I have figured out a thing or two by obsessively studying everything I can find about the subject. I am not saying this is the best filter out there, but requires very little money and no skill.
Step 1: Items Needed
You will need a 5 gallon bucket, green washable furnace/Ac filter sheet($6 at Walmart), drill and drill bit, and a water pump of suitable volume(I am using a cheap sump pump). Oh yeah....you will need an old garden hose to direct pump output.
Step 2: Bucket Prep
Turn bucket bottom up and start drilling holes in the bottom, the more holes the better. This isn't an exacting process, but small holes are better so as not to lose fish to the suction.
Step 3: More Bucket Prep
After you've drilled your hundred or so holes in the bottom of your bucket you'll want to drill lines of holes up the sides....don't skimp on these holes, they are to protect you water pump from becoming un submerged as the filter gets clogged.
Step 4: Cutting Filter to Fit in the Bucket
My first filter I cut in a near perfect size and shape to fit bottom of bucket.....it worked, but filter mat came apart after just a few months. I have since just cut into slightly oversized squares and one sheet makes 3 filters so you can quickly change them out clean for dirty. This method is getting me about a year out of 1 $6 mat!
Step 5: Assembly
Place one thickness of filter mat in bottom of your hole ravaged bucket. Place your sump pump in bucket over the filter mat. Hang bucket by rope or bungee cord where bucket is 3/4 submerged in fish tank water. Run your hose from output of pump to where you want it to fall back into water tank, and plug in your pump. Experiment with placement of bucket and return.....you can't really screw this up as long as your pump stays submerged. Everyday or two I pull the pump up and out of the water for just a second to set it on a less clogged section of filter. I try to change to a clean filter pad weekly....and just take the old one out by the garden and hose it clean.