First I'm not a DM, but i am a "World-Builder"
At different times I have done one or the other, but usually not both in the same world.
When going "Top-down" I usually start with a culture. I try to imagine what geography would reinforce that culture, and what other cultures would be their neighbors. What does the culture do well, what trades to they exceed in, what do they export. What do they not do well, what trades do the discourage. I make up organizations, religions, trade routes.
When I create "Bottom-up" I often start from a single character. I make up family and friends. A community to live in. I try to add some diverse elements (see what other races, classes, religions, can I reasonably include?). Then a mentor who inspires/encourages the character to to do more with their life. I create some background for the mentor, his community/support system, ect. I put in some chalenges. People or Organizations to oppose the character (childhood bullies, bandits, narrow-viewed leaders). Suddenly I realize that I have made 4 or 5 communities (including at least 1 city) spanning at least 3 cultures/races/nations, all built arround 1 character.