Homebrew Campaign

I'm currently in a campaign as a player and we are nearing the conclusion. I will most likely be taking over as DM next campaign and was looking at doing a homebrew campaign. I have what I feel would be a solid start but how do you go about preparing for session after or would it be prudent to wait to see where the players go and plan from there and essentially go session to session?

For a homebrew campaign, it is important to have multiple tiers of enemy planned and some vague idea how players will transition from one of these enemies to another. For instance:

1) the campaign starts with fighting Sahaugin, Merrow, and Pirates in a coastal / sea-faring adventure. During this part they find / come across a treasure map that leads them to the next part.

2) the campaign turns into a survival / dungeon crawl through dangerous tropical forest to a lost temple, through the monster-infested & trapped temple to find the pirate treasure. Removing the treasure from the temple unleashes a Lich / Curse that become the villains for the 3rd part.

3) The Lich/Curse release an army of Demons that start spreading across the land and the curse blocks out the sun. The party can go on side quests and ignore this threat but eventually the curse will catch up to them and they will have to assault the demon/undead-filled fortress of the Lich with its many magical traps.

4) Destroying / defeating the Curse reveals that some evil god is behind the whole thing, and now the party have to prepare to fight an actual god...

I wouldn't plan too far ahead. Plan the village where the adventure starts (keep it small so the players need to explore beyond it) and the first adventure. See what sparks the players attention, what motivates them, and then plan beyond that. Too many DM's over plan or create a campaign based off THEIR interests rather than the players. 

My advice is to survey your players (come up with a list of possible themes for a campaign) and then ask for their feedback. From there you can begin to plan. Session 0 is crucial for the players to make their characters fit the campaign and for you to develop hooks and threads with the characters in mind. 

If you do come up with a series of adventures or use a published adventure/campaign,  change it to suit and don't be scared to go off script to make the game more enjoyable. 

It really depends on how much experience you have as a DM or as a player, and how much your players have. If it’s your very first time I’d go for a shorter campaign, I understand why you wouldn’t want to just do a one shots, I personally find them a tad boring. Always remember that it is a collaborative story you’re making, and that runs both ways. The players need to respect what your plan is, but you also have to respect what they want to do, though a little guidance isn’t terrible. I’ve had numerous plans ruined since I didn’t account for them choosing something completely different. A lot of this depends on your personal DMing style, every dm I’ve seen has done things different. Some have a very “Yes, and” style (where they just follow and add on to the player’s actions and letting them create the main story), while many others, myself included have made a central plot and let their players work their way around it. Figure out what you prefer and roll with it. For planning sessions, I feel the same rules apply. Plan as much as you want to and feel necessary. I don’t plan a lot (mainly cause I forget to) and fly by the seat of my pants a lot, but it is fun for me, though for some DM’s I know it’s not. Essentially, just look at what the players do, and add onto it while keeping to the main plot (if that’s what you want to do). Most of all, have fun. Dming can be stressful so I hope you enjoy it.