First and foremost: Freestyle is doing tricks on a bike. It's kind of like saying "How do I change my skates into figure skates"... Your bike was designed to do 'tricks' on it - that makes it a freestyle bike. If your bike was specifically designed to do dirt, vert, street, or flatland... It would STILL be a freestyle bike. The different bike at the same size is a BMX bike (typically add 'racing' to the end). Freestyle is actually completely different than BMX and the bikes have very few similarities between them. As racing bikes are designed with a heavy weight concious approach.
Anyway... FLATLAND BIKES...
A pro level flatland bike will typically be a shorter frame than street/dirt/vert counterparts. This is dependant on the size of the rider though.
Unless you choose to ride brakeless, they will almost always have front brakes. Many riders don't use back brakes. Many riders also don't do back wheel tricks, which makes no sense to me.
The stem is typically shorter
The frame is often built less sturdy, but lighter.
A freecoaster will be on the bike.
The freecoaster can allow the rear sprocket size to be as small as 11 teeth.
The small rear sprocket is usually paired with a small front sprocket and flatlanders often run easier pedalling - so front sprocket may be 24 teeth in size.
Cranks are usually shorter, but more street riders are taking the flatland mentality with this. 160mm or so.
Tires usually are run around 100PSI.
Handlebars often have a lower crossbar and design that gives more room.
Parts are designed without sharp edges to prevent knees and shins from getting trashed.
Many riders use nylon (NOT PLASTIC!) pedals to save our shins.
Some riders use plastic seats to save weight. My seat gets trashed about every 2 months from upside down tricks.
Aluminum pegs are the norm.
Good brakes are a near must!
Now, keep in mind that almost any part you ride, you can do flatland with. It's just when designing a part SPECIFICALLY for flatland, there are little things you do to make it work a little bit better. You can do flatland on a mt. bike. You can do it on a wheelchair... or a scooter. Definitely you can do it on a street/dirt/vert bike! But, the front brakes are the single most important thing to add to make it really work. The rest of the stuff is just design improvements to help make things go easier.
Check
www.flatlandfuel.com for all the world needs for flatland.