Just bookmark the .onion link for Dream Market in your Tor browser. If it's ever down, they have tons of mirrors. Google can always find a working link for you.
You can buy bitcoins from Coinbase, my personal favorite and probably the most secure. They operate exactly like a bank. Verify your identity with them and do not worry, there is nothing illegal about buying bitcoin. The more information you validate, the more BTC you can buy.
Send the coins to Dream Market. Find your wallet address on Dream Market (constantly changes) and paste it into the transfer page on Coinbase. They will bounce around the world millions of times, to the point of being basically untraceable.
Only put on the account what you're ready to spend immediately. If Dream Market is attacked, you lose all the money in the wallet. Courtesy of your boys in blue, who will probably use it to buy shinier cars.
Never buy from a vendor with less than 4.8 stars (I'm serious, have crazy high standards and you will not be disappointed).
Always ALWAYS encrypt your address. If you fail to do this, you will get caught. You will go to jail. You're caught the minute you send plain text, even if you don't hear from anyone for years. Even if you don't get caught, this is a massive fuck up that will always potentially catch up with you.
There is a small learning curve to PGP encryption. Luckily Dream Market let's you check a box to auto encrypt your address. Literally just a click. Spell every line in ALL CAPS, even if the vendor doesn't ask in his info page. Otherwise your package might not show up.
I also don't advise buying overseas. I have no opinion of Trump other than he's really fucking up my regular big ticket orders from the EU. I might also blame Theresa May but almost every world leader is following Trump's disgraceful tactics, so screw em all.
Try to find domestic deals. You can filter to your country only. Generally you'll pay more for domestic... this sucks but it's still price competitive with any legal dispensary.
That's all the advice I can think of that a more detailed guide might overlook. The rest is just following steps that become very easy to remember.
Oh also, don't buy guns. If you're stupid enough to even look for weapons sales online, the FBI and ATF will make it their personal mission to rain hell on you. You can buy 10 truck loads of heroin but if you buy a single pistol you might as well just cuff yourself.
Hope this helped. It's really easy once you get the hang of it, good luck friend!