Done when

Query 1: 6 users, 10 published posts.
Query 2: alice, bob and dev have 3 each, carla has 1. erin and frank don't appear.
Query 3: "Indexes: a field guide" (5), "Why I switched to Postgres" (4), "Postgres from a frontend dev" (3).
Query 4: erin and frank.
Query 5: 6 rows. alice has 5, erin and frank have 0.
Query 6: sql (6), beginner (4), postgres (4).
Query 7: post 1 has 4 likes and 3 comments, post 3 has 5 likes and 2 comments. Your first attempt probably said 12 and 12.
Query 8: erin and frank.