This is the link to the thread (mentioned by Tim above) with photos of what the bumpers are supposed to achieve:
https://trombonechat.com/viewtopic.php?p=189479#p189479
and here's a link about the tricky business of getting alignment:
https://trombonechat.com/viewtopic.php?f=13&t=27035
You will see the term "witness marks" in the second link. The should be TWO sets under the threaded cap of your valve. One set indicates when the valve bearing has been set into its "reference" position in the valve shell. If this is not dead on the other mark is meaningless.
The second set of marks cross the bearing surfaces between the rotor axle and the actual bearing hole. IF the first set of marks is correct, then a SINGLE mark on the bearing hole should line up in TWO positions: valve open, and valve closed.
The diameter of the valve axis is relatively small. It is EASY to make those marks a few degrees off and not be easy to notice.
That is where a borescope comes in. I had EXCELLENT alignments from techs before borescopes were generally available. I get that alignment much more easily now. And my favorite tech (at over 90 years old) makes regular use of one.
The reason is simple leverage. The visibility of misalignment increases proportionally to the diameter of the place of measurement. If the valve axle diameter is 1/4" and the valve diameter is 1", then it will be four times easier to see a misalignment at the outer circumference of the valve.
As noted in the second thread, most of the corrections are done by shaving the bumper.
Other things to keep in mind that I don't recall from the thread:
1) bumpers may "settle in." This was VERY true in the good ol' cork days. Depending on the synthetic it can still be true. If you cut JUST right on the first pass and the bumpers settle in, you'll need to put in new ones after they settle. I've seen a couple shop tricks that avoid this, but lack the skill to do those tricks.
2) bumpers DO harden. They have to be, to some degree, compliant. Compliance usually implies some kind of volatility. That being said, I have some very old neoprene and buna seals that still work fine. But some rubbers will suck up your valve oil and find interesting and unpleasant ways to decompose. The stuff from the music supply houses is time-tested to behave.
So, maybe order more than one set? Keep spares in your case for WHEN you lose one someday? Watch your teacher carefully to learn the technique?
Enjoy the learning experience!