If you're using a replaceable plug then you can adjust it as required. If it was an old-school starfangle nut then it would be more critical to get it right the first time.

---

Less is more.

This is a pretty novice level fix, but we've all got to start somewhere. I'm not suprised to hear that Planet X doesn't have specific instructions for this.

In my early days of working on bikes, I relied heavily on Park tools instructional guides to help me think through the servicing steps. In your case, the relevant article is here, in step 8: https://www.parktool.com/...ce#article-section-8

Here's another link to a video on on the matter:
https://www.youtube.com/watch?v=ZK5u5ioZU8M

Begin by removing the adjusting bolt in the center of the steering column. Next, remove the top cap. There may be a star-shaped nut or other fittings inside the steering column. The bolt threads into this fitting and pulls on the fork against the headset bearing surfaces, which acts to tighten the adjustment. Note the height of the steering column relative to the stem. It should be about 3mm (1/8") below the level of the stem. The stem needs to press down on the spacers in order to adjust the bearings. If the steering column is level with the top of the stem, another spacer is needed below the stem.

1. Remove bolt and top cap to inspect steering column. Lubricate adjusting bolt and reinstall cap and bolt by hand only. Do not tighten.
2. Loosen stem bolt(s) that secure stem to the steering column. Lubricate these bolts if they are dry. NOTE: Do not lubricate inside stem or on steering column surface.
3. Wiggle the stem side to side to see that it is loose. If the stem is jammed or rusted frozen to the steering column, no adjustment can be made.
4. Align stem straight to wheel and gently secure the top bolt. Stop when any resistance is felt.
5. Tighten stem bolt(s).
6. Check for play by pulling back and forth on fork. Turn the handlebars in different directions while checking for play. There may be play at this early setting. Use care when grabbing suspension forks, because the legs may have play. Grab upper portion of fork.
7. To adjust bearings, loosen stem bolt(s).
8. Turn adjusting bolt in center cap only 1/8th turn clockwise.
9. Secure stem bolts, check for play again.
10. Repeat adjustments as above until play disappears. Remember to loosen stem bolts before turning adjusting bolt in cap.
11. Check alignment of stem and tighten stem binder bolts fully.

NOTE: Another test of play is to place the bike on ground and grab the front brake tightly. Press downward on the handlebars and rock the bike forward and back. A knocking sensation may indicate a loose headset. In effect this does the same thing as grabbing and pulling on the fork. However, play in the brake caliper arms may also cause a knocking. Front suspension forks may also have play in the legs, which can cause a knocking.If the adjustment seems very tight, there may be other problems in the headset. Bearing surfaces may be worn out, or the ball bearing retainers may be upside down, or a seal may be improperly aligned. If play always seems present no matter the adjustment, the steering column may be too long for the stem and top cap. Add spacers beneath stem in this case.