I would recommend these tips for making a KotH map:
1. Make the capture point first. King of the Hill maps revolve around the control point. If you can get a fun point down, then playtesting will be more fun and you'll be excited to make the rest of the map.
2. Try to use the viaduct formula, or ideas from it. I'm going to assume you know that and I'll move on.
3. Build only one side of the map, and copy it over to the other side. That way if you need to make changes, you can either delete the undeveloped half or copy props over using a skip brush.
4. Look at other TF2 maps and get a sense of what you want your map to be, and how it will be different/stand out.
5. Anything works! Go crazy so long as your idea is sensible. However keep in mind scaling, new players, and a viable way for each class to play on the map.
6. If you ever get stuck, feel free to ask the forums or your fellow players for ideas or what you should do. Your biggest enemy is demotivation! I'm sure you have creative ideas, just don't lose them.
Minor Tips:
Use dev textures early on, so that detailing isn't a hassle later in development.
Keep in mind the significance of alpha, beta, and release versions. Don't get too ahead of yourself.
If you need, take your time. And don't get discouraged if things aren't going as quickly as you imagined. It's a slow process.
Get lots of feedback, and try to get a playable version tested on a server or the TF2Maps.net servers.
Join the TF2 maps steam chat! There's tons of helpful people that are on very frequently.
Use ABS! It's super helpful, and almost mandatory.
So those are some tips from me. I'm sure there's more, but that's everything I've figured out or been taught in my roughly 2 years of developing games and using hammer.
