Send the bitcoin you want to double spend and instantly double spend from this site (without awaiting confirmation). You own this address and private key.
This is the bitcoin that you will later be able to pull back to another wallet if you choose. Usually, you have a 5-10 minute window to do this. The best chance of success is when Bitcoin traffic is high.
The first bitcoin you sent will be rerouted back to you or another wallet, minus a 2.5% double spend fee. This fee helps support the hacker community with exploit bounties and other awards.