Shortcuts are exactly that, shortcuts. For want of a dollar it’s not worth taking a shortcut. I’m sure you’re tired of hearing me negate your approach but I’m speaking from experience. Do it right the first time. Remove the head, send it out to be inspected and rebuilt. It’s not a lot of money in the grand scheme of things. Rebuild the head, install a new headgasket, new head bolts (yes new head bolts. The cheap bastards will disagree but they aren’t going to start a go fund me page for you when they realize they gave you bad advice). You’ll be happy, your engine will be happy. Your wife will be happy and your dog will be happy. Plus I will be happy knowing that you listened. And there you have it! That’s how you replace valve stem seals without taking the head off.
After you get that all figured out, you’re ready for the real fun! I try to set the tool up to be at the same angle as the valve I’m working with so it’s pushing directly downwards. You may have to play with a couple of combinations on the tool to get the right angle and the right amount of stroke (especially on the intake side where the manifold tends to stop the tool from compressing the spring enough). Here’s why I prefer using rope in the cylinders over the compressed air method: Most write-ups say to fill the cylinder with 100psi to hold the valves in place while you compress the spring. I believe the stock intake valve diameter on a 1.6L is 31mm. A quick calculation tells me that the 100psi in the cylinder is putting about 117lbs of upward force on that valve. I can tell you on at least half of the valves it took over 100lbs of force to crack the retainer loose from the keeper. I certainly don’t want to break the seal of the cylinder and have my valve go plowing into the top of the piston with my weight behind it. At any rate, to each his own.
What you want to do is pry downwards on the tool, pushing the retainer down the valve stem and freeing the keepers. The keepers will probably stick to the valve stem, so you want to have a pick handy to flick them off. Once they are free, slowly release the tool until the spring is fully uncompressed. I’ve included a rudimentary drawing as well. (Base picture from TheAutoPartsShopDOTcom modified with my mind blowing MS Paint skill)