Every week you should do a partial water change. What percentage you do depends on how heavy your stocking is. Try to do enough water changes so that your nitrate doesn't get to more than about 20ppm above what comes out of your tap. You have to find the right level and frequency of changes for your own particular set up. I do 50 or 60% a week on all my tanks, but if your tank is very heavily stocked you might need to do larger than that, or twice weekly ones.
Get a piece of pipe, or a gravel cleaner, and a bucket. Switch off all the electrics before you start!
Start off the syphon (I just suck on the bucket end of the pipe, but you can immerse the whole thing in the tank, turning it round to get the air out, and then move the free end of the pipe to the bucket, with your finger or thumb over the end if you'd rather), and suck up as much 'mulm' as you can. It might be easier to have two or three buckets so you can carry on gravel cleaning without having to stop and empty your bucket all the time!
Once you've got enough water out, wipe down the sides (I use a new washing up sponge; a cheap one that doesn't have any soap or detergents impregnated in it) to get rid of any algae. You can also
gently swish out your filter media in one of the buckets of old tank water to stop it getting clogged.
Then refill the tank with warmed, dechlorinated water ans switch everything back on. I normally save my frozen food feeding day for after I've done a water change, just to say 'sorry' to the fish for messing with their home
You don't ever need to totally strip down tanks to clean them, unless you've had disease or are changing the substrate, times like that.