Ex-NBA hooper and current weed entrepreneur Al Harrington has some solid advice for any newbies looking to celebrate the 4/20 holiday this year ... telling TMZ Sports it's pretty simple -- "get high as f***!!!"
Harrington knows his stuff -- the guy built an entire company around his love for Mary Jane ... serving as the CEO of the Viola brand.
He's doing pretty well for himself, too ... with the company valued at over $100 million.