Adding weight to flies is often accomplished using lead-free wire in different diameters; usually 10, 15 and 20/1000”. Lead wire is still available but I no longer tie with it for environmental reasons.
In general, smaller diameter wire is used on smaller hooks and larger diameter on larger hooks. If possible, you want the diameter of the lead-free wire to closely match the diameter of the hook wire.
I don’t like waste, so I’ll secure the wire by first grabbing the bitter end at the back of the hook. Plunger-style hackle pliers make doing this much easier. I’ll then take touching wraps forward up the hook shank, usually somewhere between 8 and 12, before helicoptering to break the wire off. This way there’s absolutely no waste. Then I’ll do my best to tuck the tail in close.
On beadhead flies, the wire does double duty by adding still more weight and helping to stabilize the bead when pushed all the way forward.
Additionally, when covered with thread or other material, the lead-free wire can be used to build a nicely tapered body on the fly.