A seat post is the last part I would make if I had never made tubes.  A broken, sharp seat post where the sun don't shine is not good. If you do decide to do it I would:

Calculate the thickness of of the carbon fiber required.

Lathe down an aluminum rod as your mold.

The diameter of the aluminum rod plus the calculated thickness of the carbon fiber will equal your outside diameter.  So if you measure the outside diameter of a seatpost and subtract the required carbon thickness that is what you need to lathe your mold to.

Layup prepreg or OOA prepreg alternating between vertical, horizontal and 45 degrees both ways.  I would build up the lower section where it clamps in, several layers thicker.

Bag it and cure in autoclave or oven.

Trim to desired length.

You wont have a glossy part, but you can clear it if that is the look you want.

I would analize from the stress type point of view.

thank you for advice. 
also percentage will help.

Like Hanaldo said you should at least match the existing 1.3mm alloy tube and then I think that considering you are doing this wet laid I would still go for 2mm.

I work a lot with carbon tubes and boats. Most tubes 20mm dia to 70mm dia are usually around 1.6mm wall with about 70 % fibre longitudinal and 30% in the hoop orientation. Smaller tubes 10mm-20mm dia are usually 1-1.5mm wall.

agree, but my momentary alu seatpost has only 1,3mm thick walls. so if i go with 7-8 layers plus 1 spread tow finishing. it will bo close to 1mm. what should be enough.

i was thinking of wet layup on bladder. and than whole mould in vacuum bag. hopping to get less bubbles in part.

Typically 1000g of carbon used with vacuum or shrink tape is 1mm so you would need around 2000g for 2mm.

7 x 100g is just 700g and less than 1mm thick, not very strong especially if this is not the usual pre preg that most tubes are.

agree. it is potentially a danger. i don't wont to go to extreme. but i would still like, if i do something from carbon, that will look better than alu and at same time save weight.
but not to go in extreme. that's why i wont made handlebar
i think if i do same layup trough whole length, and if breaks, it will break just over the frame. there is the most stress.

i was thinking using UD carbon 100g/m. 5-7 layers. and every layer little bit different orientation. that all of them cover bit less than 45 degree.

My main worry is that it's potentially a dangerous thing to make, so you don't want to take any chances with it. Maybe it would be safe though if you deliberately make it weakest just over the point where it leaves the frame so that if it does break it will go there without leaving a shard sticking up. If you can do that, it may be okay to risk going for a big weight saving.

i agree with Hanaldo.
its no point of putting all that work to make same weight part as metal. but thanks David for answer.

and as i read is kevlar only crack arrestor. and if not cured at exact right temperature it doesn't bond. so i think i will skip it.

i was thinking kinda a same direction. same thickness. but if i would like to go more extreme.
carbon fiber have twice the strength as aluminum.
so in theory it can be halve the thickness of alu ? of course its not that simple...
and of course depends on layup and type of fabric.

I would disagree with building it to the same weight as the metal part it is replacing, though to some extent that does depend on the mechanical strength required. The general rule of thumb is to build it to the same THICKNESS as the metal part it is replacing. So if the original part is a 2mm thick steel tube, then a 2mm thick carbon tube should be ideal.