How do countries and states that have legalised cannabis manage to tax it ?

They can't.that's the simple answer. They should burn down the houses that cultivate them !

California stands alone as the only state where legalizing recreational pot has resulted in an even bigger black market for it. This is because they set the tax rate inordinately high here. Apparently in other states they got it right and are reaping the benefits of tax revenue pouring in from this product. 

In my weed friendly state, consumers can grow up to 4 plants per person. They can legally gift it to other adults but they are not allowed to legally sell it. There are people who break every law on the books and I'm sure some people break this law.

It is not legal in my state, but it is in the neighboring state. They require a license and tax the dispensaries that legally grow and sell it. You are allowed to grow a small amount for personal use, but you can not sell it without a license. 

Alongside the commercial cannabis industry, some jurisdictions permit people to grow a limited amount for their own use. Most people will find it difficult to do successfully.

The law prevents them from selling privately.  They can still be fine and go to jail if caught.

Producing and selling  goods without collecting tax is illegal.   Those who violate tax laws are subject to fines and possible imprisonment.  

I hope that helps.