What the achievement asks
Collect five grey Sandworm Scales, one of each colour: Cobalt, Silver, Emerald, Ruby and Onyx. Five points, account-wide, Horde only.
They all drop from the same creature, the Ravenous Sandworm, at a low rate. There is no skill to it and nothing to find. It is a waiting game with one trap in the middle, and the trap is what this guide is really about.
You need the Sandfishing world quest
The scales only exist while the Sandfishing world quest is up in Vol'dun, and it is one of the rarest in the zone: gaps of over a month are normal, and people have waited six weeks for it.
When it is up, entering the quest area gives you a Sandfishing Pole extra action button. Using it out of combat summons Ravenous Sandworms, and the scales drop from them.
The trap
The world quest asks you to loot eight Sandworm Fang. The moment you loot the eighth, the quest completes, your fishing pole disappears, and you can no longer summon anything.
That is how most people lose an evening. The fangs are the quest objective, and auto-loot takes them without asking.
Turn auto-loot off before you start, and loot only the grey scales. If you use a loot addon that grabs everything, turn it off too.
Some people use a raid group instead, on the grounds that quest items do not register there. It works for many and has been reported failing for others, so treat it as a convenience rather than the safeguard. The reliable protection is auto-loot off.
Even if you do complete the quest by accident, you are not finished. Scales drop for anyone who tags a worm, whether or not they have the quest. Stay in the area and keep tagging what other people summon.
Fishing properly
The pole has a cast time and only works out of combat, which sounds limiting until you realise you can get ahead of it.
Spam the button. You can start roughly three casts before the first worms arrive and drag you into combat, which brings four to eight worms at once instead of one. That multiplies your drop chances per cycle, and it is the single biggest difference between a ten minute run and an hour.
Then kill them, loot them, move a few paces, and cast again.
Check every corpse. The worms burrow and surface as they come, so they end up further apart than they look, and the area loot radius is smaller than it seems. Scales are missed this way rather than not dropped.
What to expect
Duplicates, a lot of them. Runs of the same colour four or five times in a row are normal, and the order is pure chance.
Most people finish in ten to thirty minutes. Some finish in seven. A few have gone over an hour on a single stubborn colour, and there is nothing to do about it but keep casting.
Grouping helps, because more people means more worms in the same patch of sand. If someone else is clearly there for the achievement, invite them: you both summon, and you both tag. Keep the group in one place rather than spreading out, so you are not stealing tags from the people next to you.