You don't need to, that is why Monero is the far superior cryptocurrency for darknet market related activities. In its most simplist form, the blockchain by design is private. You can purchase Bitcoin by any means, even with your own identification. Once you have purchased your Bitcoin you can exchange it over to Monero. If you have purchased Monero directly you can ignore steps 1 and 2 and simply send the Monero to a new wallet before funding your order. This is how the flow looks;
Buy Bitcoin -> Exchange Bitcoin for Monero -> Recieve Monero in a holding wallet -> Send your Monero to a new wallet -> Fund order
Once your Monero has been sent to the last wallet, you are 99% safe to make a purchase anonymously. This is because once your Monero goes from holding wallet #1 to holding wallet #2 the chain has been broken. Up until Monero wallet #1 there is a very small chance of it being traced back to you, once you send it to the second Monero wallet there is no way to track where the funds were sent there on unless there is a huge oversight or bug within the Monero cryptocurrency.
This is where things get tough, I know for a fact very few people use Monero and very few people even clean their Bitcoin. Most people make purchases for personal amounts thus don't see the point in going to extreme lengths which is true, as personal use buyers do not get caught very often. Should you be careless? That choice is totally up to you. The absolute best way of cleaning your Bitcoin is by doing the following;
Purchase Bitcoin anonymously -> Funding your order
Please note, purchasing Bitcoin anonymously means you mask your browser fingerprint, do not use any payment cards/methods that can link back to your name, or use a physical Bitcoin ATM where your face is not recorded somewhere. Purchasing directly for cash is also a reliable method if you want to wait longer to aquire your Bitcoin.
It is a lot easier to buy Bitcoin anonymously than it is to buy Monero anonymously, you can actually replace Bitcoin with any cryptocurrency, for example Bitcoin cash. This is by far bulletproof, and if you purchase large amounts for reselling this is what you should be doing.
Purchase Bitcoin using ID -> Exchange Bitcoin for Monero -> Recieve Monero in a holding wallet -> Send your Monero to a new wallet
This again is a very good method, but if you're converting to Monero you might as well just purchase your item with Monero. This is useful to pay vendors who don't accept Monero (which is bad to begin with) but due to price fluctuation and the lack of knowledge you can understand why some vendors do not accept Monero.
Bitcoin tumbling
With blockchain analysis companies receiving millions of dollars a year to assist the government, do not even bother wasting your time. Yes, it makes it harder to track the origin of your funds but really technology can automated that process which cancels it out. All you are doing is risking your funds with some sketchy tumbler operator, and paying fees for a false sense of security.
Remember, Bitcoin is as anonymous as you make it. Using a VPN or routing your wallet through the Tor network will do absolutely nothing for you, the number one way vendors, customers, and markets get deanonymized is by following the money. This tutorial is not intended to fearmong or shill Monero. I am trying to better educate the community so you can make a decision yourself on what lengths you want to go to, to remain anonymous.