Here is a drawing of the parts used on the valve and the assembly. Note the 2 grooves on the valve stem - the lower groove is for the small O-ring seal. The top groove is for the split locks.
Compress the spring retainer below the 2nd groove, install the O-ring seal, then the split locks, and back off on the spring compressor.
Check the split locks to ensure you see a little space between them - you want that. They should not butt solid together or something is wrong.
On aftermarket valves, you typically will not see the second groove for the O-ring seal, so you don't use them. However, you want some type of seal and with aftermarket single groove valves, you want to use the valve guide seals which means you will have to have the guides machined for them to be used.
The other option is to have the second groove machined into the stem by the manufacturer. I do believe companies like Ferrea who manufactures their valves could do this if you did not want to cut down a set of heads for the valve guide seals.
Another thing to note is the oil shield which goes over the spring and under the spring retainer. This is designed to help keep excess oil off the valve stem and thus extra oil going down the valve stem and making your engine smoke out the tail pipe on start up (which can also be a sign of worn out O-ring seals).
Some will leave these off. Probably not a good idea as the shield fits over the spring and the retainer than fits into the shield. My guess is this keeps the spring centered and from walking around. I have never checked, but just putting the retainer on top of the spring may not center the spring as with the shield being used.
Now with the heads I had rebuilt, I used the valve guide style Viton seals and had the guides cut down and installed the bronze valve guides. My machinist ordered new springs as well as the matching spring retainers. With this set-up, no oil shield is needed or used.
The oil shield might also need to be removed for clearance purposes if you have a high lift cam, but I do not know at what lift (if any) that that would occur - so always check you clearances.