With difficulty

It's a right pain to vacuum when you have heavily planted/decorated tank... what I tend to do is vacuum all the bare gravel first, go as close to my plants as possible (it's ok to leave a bit of crud round them, it's nice and nutritious), and every couple of weeks I take out my bogwood, move my stones (also, as I have java fern and anubias which aren't rooting plants, I sometimes shift the plants around a bit too) and vacuum under them. I find under things is where most of the mulm tends to accumulate; so I'd advise you to remove the ornaments for cleaning every couple of weeks, if possible. It also helps if you sink the ornament right down in the gravel (if it's not too deep a layer) so that there is no gravel trapped underneath it, but around it instead.
Remember, there is no need to vacuum all or even most of your gravel every week. Concentrate on just cleaning one half of the tank really thoroughly, then do the other half next week
