Isn't the lower spring seat inside diameter larger than the valve seal? If not then I would be for putting it on then installing the seal to keep from messing it up. I think the last kit I got for seals had some hard plastic tips that went on the valve guides to keep from cutting up the inside of the seal. The springs should be large enough to go over the seal and not cut into them. Been a while since I did the head rebuild thing. Memory is going as I am cramming too much new info in the confined space all the time.
The valve spring seat should go on first.
The plastic caps go over the ends of the valve stems not the guides. Just be gently in moving the seals over the valve stems and it should be fine.