These people don't know what they are talking about. I've brought psychedelics off of several darknet markets. I will answer your questions.
First of all don't fuck around with anything other than psychedelics on there. As a psychedelic user I know there is a world of difference between a psychedelic like 2-b and heroin for instance. Psychedelics are non-addictive, except for maybe the dissociatives like ketamine, but heck even that is legal and doctor prescribed. If you decide to take the plunge then you must do a lot of research first to make sure you are safe given the amount of the drug that your going to injest.
First of all you need to download Tor Browser at
https://www.torproject.org/download/
Set your tor browser to "safest" in the settings.
Then you need to find an underground market. Many of them have been shut down. All you have to do is google reddit top darknet markets or top darknet markets and choose the one that starts with an "E." or whatever one is seems most legitimate.
All the darknet markets I've brought from have been taken down by LE. But the vendors just go to another marketplace and start new. LE usually don't go after the buyers espcially of psychedelics. They are the last people in a long chain of others they would rather go after.
If you have any questions I'll be glad to answer.