I would practice on old bike frames. I happened to have a 4130 Trek that broke. By the time I'd cut it up into pieces a few inches long and welded them back together again at various angles I just about had the ability to make my first new frame. I TIG everything but you can probably heat the lugs off an old frame. Or make some lug-like things for practising. The steel alloy doesn't matter but the thickness does, especially for TIG: you need to be able to handle thin walled tubes.