Place empty bucket beneath your fish tank, the gravel cleaner is gravity powered and water only goes down hill.
Switch off your heater and depending on whether you will get wet if you don't your filter.
Stick the end that is just a pipe over or ideally in said bucket.
Stick the end with the funnel into fish tank water.
Prime the vacuum. Different models have different techniques.
Suck end in bucket and hope you don't get a mouthful of fish crap.
At this point water is now flowing from tank to bucket, so stick funnel in gravel vertically.
The lift and dunk method gets right into a particular area, the hover and suck covers a greater area, but doesn't lift as much crap.
Remember your bucket is a finite resource; stop sucking before it overflows. (ie:Take funnel out of water).
Replace water removed from fish tank with fresh, switch on heater and filter.
Take bucket of dirty water and feed your roses, (it's full of nitrogen).
Clean funnel end of vacuum under tap as it will have fish crap stuck on it