Get a notebook. I use a standard legal pad. There are three parts to every post, no matter how long and detailed. They are (description/action/reaction). Make a note of every action you need to respond to, leaving a space to write in YC's reaction, including dialog and physical actions by the other player. Thoughts and emotions are off the table unless they're couched in a descriptive way like a blush, pause, hesitation.
Once you have all of that, you can build YC's actions. RP is like improve; it needs a (yes/and). You've acknowledged or responded to your partner's actions; now you have to decide what YC's actions are going to be. You're looking to give your partner enough to write about. That's the basic rough draft of your post. The reactions should be in the order of how your partner wrote them, but you can slide your actions in between where they seem appropriate. Keep it simple these are just notes.
Start adding the descriptors as you write the post with the outline already laid out. Pay very close attention to details. You might have to go back ten posts to find what the hair of a particular character is like. In the notepad, keep pages to jot down weird thought bubbles that might be exciting plot points for some time in the future. Writing complex posts becomes a formula after a while. The actual work depends on how long you can sit at the keyboard, and don't expect to do it all in one session.
Here's the thing, If you and your partner are free all day, it can be a lot of fun to ping pong one or two paragraphs back and forth. If, however, one of you is super busy, that's where the big 2,000 word posts come in handy because they can be written over a week as you find time to shoehorn in a bit of writing.
When writing a post or description, you can play a little game by slowly revealing something. Let's say you have a dead body hanging in a meat locker. Start by describing the shoes hanging off the floor and then draw your partner's eyes slowly up to the face. Try it in another way, keep the dialog casual, friendly, as you walk their character to the edge of the building without any indication of what you're already planning. Then when they walk to the edge with you because we don't godmod, in the next post, you have the push.