I have always speared with my float attached to my speargun/railgun. I recommend starting with that* - it is simple & safe & you won't loose your speargun. But I think some folk, esp. who spear with a partner, start with it attached to their belt. There are advantages & disadvantages to both approaches. For example, attached to your belt you would be easier to find if something went wrong & your belt would be easier to find & retrieve if you had to drop it (or accidentally drop it). Also dropping your belt would free you of both weights & float-line, provide you weren't tangled in it.
*I started mainly SA Rob Allen gear & that's what was suggested to me when I started by an experienced SA spearo. It works pretty well and I'd hate to loose my speargun, so I have stuck with it. This year though, I have it in mind to try some different approaches and have ordered a few new items to allow me to do that. I will likely take it carefully and try only one new thing at a time.
e.g.
1. Float-line attached to reef-hook attached to speargun
I think this might be a simple but useful addition to my current simple rig.
http://www.spearfishing.co.uk/shop/spearfishing-accessories/spearfishing-reef-hook/
2. Float-line attached to ring on my weight-belt + drop weight
This would be a more radical change for me. I am hoping to use a 500g clip-weight for that, so that I can drop the weight & float line as a marker for me to dive around. Several times that would have likely helped me relocate hot spots - but the tide/current is often strong enough to ensure a dropped weight would likely not stay put unless perhaps dropper in thick weed and even then.... Perhaps better saved for one of those sheltered locations on a quiet day?
e.g.
http://www.scubastore.com/scuba-diving/omer-belt-d-ring/5587/p
http://aws.tradeinn.com/d/59/594562/spetton-inox-carabiner-quick-release-lead-500.jpg
3. Float-line attached to ring on my weight-belt + speargun
This one bothers me most, as some care will be required to avoid increased tangle/snag hazards. Would likely use
Orca clips and/or regular old climbing carabiners (non-screwgate of course) or clip-weight or perhaps just a simple quick release knot on the belt or perhaps reef-hook? TBD
http://www.scubastore.com/scuba-diving/picasso-orca-clip/603063/p
I think trying any & all of the above changes will warrant additional care, any significant change to your rig does.