really you should get it done professionally you could try your lbs and see if they do it, or perhaps the local garage might be able to help you.

If you are really set on doing it yourself then you need to either spend a lot of time sanding all the purple paint off, or get hold of a sand blaster, either way you need to remove all the old paint. Then you need to get hold of a paint gun, and shoot a coat or two of primer, then shoot the colour, depending on whatever paint you use, you may need to apply a clear protective paint over the top, most good quality paints wont need it tho. make sure it dries for a couple of days and aply evenly all youll get cursed drips dried into your paint job.

My advice tho - get somebody professional to do it, ( a seat post isnt very big so it cant cost that much) espeially if you've never used a blaster or paint gun, coz otherwise its just going to look ugly and messy, and youl end up with drips and all sorts, i know iv'e been there and it really looked bad.

Either way good luck