One method that has always helped me is making a basic roadmap. It may seem like having a start and an end will be enough, but when you are actually roleplaying it out, things might go slower then you expect or players might start to falter a bit on their journey. You need to be able to look ahead and know where things are going even when your players throw you curveballs. That's the beauty of roleplaying, it can be filled with all sorts of surprises.

This isn't about railroading your players. The good thing about having a basic roadmap is there is plenty of room for your players to do their own thing. You are the driver and you have a destination, but your passangers can stop to smell the roses at any time, or you can take a detour they like the look of. You won't get lost because you still know where you're going.

[hider=This method works for me][indent]• Get a piece of paper, put your roleplay start at one end
• Put your desired ending at the other end
• For every idea or checkpoint you want your players to experience, draw a bubble somewhere inbetween and write it in
• Finally, add any ideas that you aren't sure fit in yet, or cool things you want to do for the genre or roleplay that are still quite loose in your mind
• Are any of these events dependent on each other? For example, one only triggers as a result of another one? Connect them with a line and arrow
• Make noticable any key checkpoints that [i]must[/i] happen during the course of the roleplay[/indent][/hider]

What you'll end up with is a rough path through the roleplay, with a few floaty bits hanging around. The floaty bits can be used when and if you need them. If the players go to Point A, they might meet a gang. If they go to Point B, they might meet an undercover cop. Undercover cop is crucial to the roleplay? No problem, he's hanging out at Point A259 that the players created themselves.

Basically, at the end of the day, it doesn't matter if your players find the solution to the problem by finding a key or killing the president, they're still going to end up at the final boss fight one way or another. So you need to know what that cruicial point is and have that planned out. You can edit it a little easy to suit your players, but you need to know your key parts to make the roleplay feel like a congruent whole. 

Of course, you can wing it if that's your style. It can work really well. But sometimes, particularly if it's am ambitious idea, you'll get lost along the way and things won't turn out to the flavour of your origional idea. If you want to wing it, the best thing I would say is [b]tell your players[/b]. [b]Make it really clear[/b]. Why? because then you've given them agency to craft the plot with you. They know they have creative freedom and won't be stepping on anyone's toes. For a freeform roleplay, that can be important.

I would say that, it is very easy to get in your own head about things. As people have said, roleplays are trial and error. It's very possible to overplan things. Keep yourself in check, stay excited about your idea, and don't get bogged down in the minutia too much.

[b]Finally, the most important aspect that will make your roleplay work is your players[/b]. They are precious, treat them with respect. Say No to people who aren't a good fit for your roleplay. It can be hard, I know, but it's far better to have a good strong group of players who will work with you, be excited, be active, then saying yes to players who ultimately you know are either just going to ditch or who aren't treating you, the GM with respect or other players with respect.