You either have worn rings, or more likely hardened or damaged valve stem seals. I'm not sure one is much better than the other, in terms of how easy they are to fix.
Do a compression test to check compression. If compression is good... and also even across all the cylinders, then it's more likely to be a dodgy valve stem seal. If one or two cylinders have compression that is more than 10% lower than the others, then you could have a piston ring problem.
Whichever it is, you're looking at a head removal job. The valve-guide seals are under the rocker cover but you need the head off to hold the valve in place once you have removed the valve collets and springs.
Rings need the pistons to come out.. usually the easiest way is to prod them out of the head from underneath (which saves taking the crank out).
It's a big job in terms of labour. You will need to price in the new rings and/or valve guide seals, a new head gasket.... and also the "while you're here" stuff. It makes sense to replace the cam-belt, tensioners and water-pump while the head is being dismantled, especially if you're due a cam-belt change anytime soon, since the belt has to come off anyway... so no sense to re-fit the old one, unless it's brand new.
If it was me, I'd replace the seals and the piston rings regardless of where the problem is. Then at least you'll have everything in there "all new" so you'll get some value out of the huge bill you're about to get. It won't need any more attention for a very long time.
Ralf S.