jump super high, throw the bar super fast, I take my back foot off as soon as I jump, kick with the front foot, get it around with your wrists, keep it under you, and land on it. thats my advice
Just catch the bar completely. A lot of people tend to just catch a little bit, which doesn't matter if it's only a barspin, but it makes it a lot harder to whip right after.
Think of the bar and whip pretty much as 2 separate tricks. I keep both of my feet on during the bar which gives me more control and power on the whip, but it's whatever is most comfortable for you. Most of all, PRACTICE!!!