For the love of Arceus, don't come here looking for Diglett when there's a raging sandstorm going on. Now, a lot of the Diglett can be found by scouting the perimter of the desert. So the visibility doesn't matter too much. But there are a few in the middle of the sand, with no landmarks nearby.
1/10: From the entrance, turn left, while keeping close to the edge of the zone. Nearby, there's a Pokémon Den. It's hiding a few paces away from the den, near the tree and cliff wall.
2/10: From Diglett 1, continue following the cliff wall in a clock-wise direction. Soon, there's a boulder and some shrubbery by the cliff wall. Number 2 is waiting here.
3/10: From Diglett 2, continue following the cliff wall in a clock-wise direction. Keep going until you reach a boulder in the corner of the zone. You'll go past two tall trees. Search near the vegetation below the boulder.
4/10: From Diglett 3, stand where the gold item ball (TR21 Reversal) is, with the cliff wall to your back. Run straight ahead for around 10 seconds. It's located in the desert sand, far from any discernible landmarks.
5/10: From Diglett 4, continue following the cliff wall in a clock-wise direction. Soon, you'll reach another boulder with a regular item ball. Stand where the item ball is and turn around to face the nearest Pokémon Den. It's in the sand, a few paces away.
6/10: Again, keep following the cliff wall in a clock-wise manner. Once you reach the next boulder with a Pokémon Den nearby, go past the den and check around the tall tree.
7/10: From Diglett 6, stand with the cliff wall to your back. Ahead, there's a really tall sand dune. Go to the top of the dune to find lucky number 7.
8/10: From Diglett 7, face the entrance to Warm-Up Tunnel. Head for the tall tree and boulder between the tunnel and this sand dune. A few paces before reaching the tree, you should stumble upon Number 8.
9/10: From Diglett 8, turn so the Warm-Up Tunnel is on your left. There should be a medium-sized sand dune ahead. Go to the top of this dune to find what you're looking for.
10/10: From Diglett 9, if Warm-Up Tunnel is still on your left, there should be a boulder and vegetation directly ahead, away from the cliff wall. Go over there and check around the right for the final cause of migraine.
The tally so far is 110+. Your next reward will be after you've found all 150 Diglett. Time to roll up your sleeves, then.