I know you got a great reply already but I would love to add my 2-cents!
1. How do I reveal backstory?
Depends, to be honest. Info-dumping is something that you really don’t want to do - but this is dependent on genre. In literary fiction, or classic literature, backstory is done in an entertaining way and is told like a story in and of itself. If you decide to go this route, you really have to employ your narrative/storytelling skills to pull it off. There is also the good ol’ flashback to help you.
Another way to reveal backstory is to sprinkle little nuggets throughout the narrative that hints at something greater. Easy example: you can mention your heroine avoiding the hero for some reason; she doesn’t even want to look at his face. Then have it revealed later on that the hero is the doppelganger/look-alike of her previous love who died in a tragic accident.
Preferably, you’d want to sprinkle in information to get readers curious and lead them up to the big reveal.
2. How do I describe character interactions? Plan them out? Get ideas for them?
Okay, this stumps me but only because I probably do this automatically so I don’t really think about it. But… there are a couple ways to describe character interactions:
- Interweaved with dialogue. ***Example:*** "Yes, sir!" Jodha replied, flitting towards the exit with a mock-salute and a roll of her eyes. "Your coffee will be as you expect it." Out in the hallway, a devious grin curved her face. (This interaction shows that she is up to some harmless mischief without literally telling the reader. This is a great way to communicate a character's intentions without being too detailed.)
- Within an experience or vantage point setting: if your MC is at the beach, use the interplay of experience to form your character's interactions with the setting around them. (*With the waves lapping about his waist, Carlisle raised his head towards the warm sun's embrace.*)
It really depends on your intention for a scene. You can plan it out, if you’re a plotter or allow your characters to direct you, if you’re a pantser. It is usually a good idea to understand the emotion you want to evoke out of a scene before you actually start writing though.
You can check out Dahlia Evans’ books. I have some of them but I wasn’t able to read them yet but I hear that they are excellent.
3. How do I worldbuild?
I wish I could help you with this one but I do hope you find someone who is more passionate about this aspect than I am. Rachel Writes on YouTube has some great worldbuilding video resources.
4. How do I know if I’m doing the right thing while planning?
Your plot is just a guideline, not the be-all, end-all for your story. Writers, who have been writing for years and are hardcore plotters, know that even the best plot can go off course.
This is perfectly okay! It’s a great sign that your story wants to be told with authenticity. What I will suggest is not to plan so heavily that you force yourself into a box. Plan your story, but allow for contingencies in the plot so you can have more freedom with your writing. You can direct your characters to a point, but ultimately, they are people (albeit fictional) and they will know where the story should naturally go. Trust them to lead you.
5. How do I manage wanting to be the writer and reader at the same time?
Just don’t. Lol.
Your duty, as a writer, is to get the first draft out with as little resistance as possible. If you’re looking at your story through the eyes of a reader while writing, you’re setting yourself up for:
- agonizing over every line and not moving on until it’s perfect.
- wanting to be able to communicate what you visualize flawlessly in one go
- burn out
- being incapable of finishing a single book
That being said, you can write what you want to read but… try not to read it while writing.
It’s important to remember that what readers see is a final, finished, polished product so if you’re looking at your crappy draft through the eyes of a reader, you will be subconsciously comparing your draft to a finished manuscript.
When writing, just think about getting the words out of you as fast as you can, without regard for the way it is coming out. There is a reason why editing exists. Drafts are meant to be crappy, until you can gain experience to be able to produce a relatively clean one 
Being in the mindset of a writer allows you the overview of your story and you can better hide your plot threads so as not to reveal too much to the reader early on. You have greater control over what you hide or reveal, as opposed to judging your work from a reader’s view.
Hope this helps you!
thanks.