You'd think it would be extremely difficult searching for Diglett in a forest, but it's not actually too bad. Fortunately, the developers were merciful (for once) and only hid 8 in the forest. Still, without a guide, it can be really hard to find even this small amount.
1/8: Starting from the Soothing Wetlands entrance, go up towards the first bridge. It's next to the tree that's immediately on the right.
2/8: From the wetlands entrance, continue along the right side of the forest (without crossing the bridge). Turn left after the first stretch of tall grass. Below some yellow flowers to the left of a berry tree.
3/8: From Diglett 3, keep on following the forest until you reach the second bridge. Cross the bridge and turn right. It's hiding after the red flowers near the river.
4/8: From Diglett 4, continue heading right from the second bridge. Go past the Pokémon Den, all the way to the edge of the land. The cheeky sod is waiting towards the right.
5/8: Return to the second bridge and stand so it's behind you. Follow the path in front, as it goes around. Count the Pokémon along the way. When you go past the 3rd Pokémon, look around the right. It's before some red flowers.
6/8: From Diglett 5, keep going forward. There should be a tree nearby, along the right side. Check near the roots of this tree.
7/8: From Diglett 6, go towards the tall grass just ahead and turn right, where the berry tree is. It's hiding in the middle of the red flowers to the left of the berry tree.
8/8: From Diglett 7, head for the riverbank, where there's a view of the first bridge. The final Diglett is located below a small bush that's next to the raised, inaccessible land (near the bridge).
You should have a total of 46 Diglett now. That's enough to earn you an Alolan Raichu. Surf's up!