Trade bitcoin for altcoins on one exchange. Move altcoins to another exchange.and trade for BTC. Move to a wallet you control. Use Tor when you do this.
If you're going to do that, might as well say "trade BTC for cash and then use the cash to buy BTC." This has nothing to do with making it untraceable. Converting from one to another isn't making it anonymous, it's using alternate forms of money. Two very different things.