Does anyone have any reliable resources regarding properly/safely mixing BTC?
The main point of using a mixing service is that you want to break the link between your past transaction history and your future transaction history. So, to make your mixing in a proper way, you should learn (1) some useful techniques that might help to protect your privacy, such as coin control (UTXO management) and (2) basic heuristics used in blockchain analysis. In other words, you should train your skills of correct usage of the capabilities of the bitcoin network so that you can preserve your privacy during the whole mixing process, which includes pre-mix and post-mix periods. Otherwise, your effort in breaking the connection between your sending addresses and your receiving addresses will fail.
Quick tips:
1) Never use exchanges to deposit to or withdraw from a mixing service. Instead, use your own wallet over which you have full control. Not your keys, not your bitcoin, remember. Ideally, your wallet is connected to your own bitcoin full node, all connections to the network are via Tor.
2) Concerning coin control, UTXO, and address management. When you are sending to a mixer you should avoid at all costs merging your KYCed coins with non-KYCed coins. You don't want a blockchain analyst or other unwanted observer to know that you have other coins besides those that are already linked to your real identity. Ideally, UTXO shouldn't be merged ever, even if they aren't explicitly connected to your identity. Send them separately, one by one, avoiding the creation of unnecessary change.
3) If a mixer of your choice doesn't support connection via Tor or requires you to have JavaScript enabled, don't use it. Ideally, use a mixer that has already proven its credibility.
4) Use all possible capabilities of a mixer, for example sending to multiple addresses, time delays, etc.
5) After your mixing is done, you got multiple private keys from the mixer and sent coins to your own wallet, the job can finally be considered done. However, you should remember that coin control is important even after you have broken the link, especially, if there is a change left from your previous transactions. Do not merge your change output with any of the mixed outputs because it will make your mixing useless.