I can only speak for myself in this instance as many people follow creative templates that work well both for many and few that are specifically designed for themselves. I think there are several "phases" in the creative process that can be navigated in a multitude of ways.
For me, it starts with an initial idea that is developed in one voice. For instance, I hear a catchy melody in my head that develops into an entire phrase. So I'll grab my keyboard and jot it down before I forget it.
From that one idea I build based on voices. So say the part I have created is a strong lead. I then work all at once on a complementary bass, drum pattern and/or harmony parts. So once I have a some basic material written for all voices I intend to use, I start thinking large scale for development of the idea. I think of my voices as characters on a stage. ( This is what they pounded into my head in composition school and it works! ) When you think of your material as a character in a play, it doesn't stay stagnant. It grows and develops throughout the play and by the end it has become a new character shaped and molded by it's experiences. This is how I develop a solid, interesting line.
From those basic ideas written using a single instrument I start to delegate instrumentation for each idea. Browsing VSTs and sampling different sounds, I start to individualize the material based on the unique timbres of the instruments playing back the voices. I build phrases and begin a song structure. Intro -> Verse 1 -> Chorus -> Verse 2 -> Chorus -> Verse 3/ Bridge -> Chorus -> Outtro (For example, I don't always follow a pattern like this.)
Once the track has taken a shape developmentally it's time to balance the voices so they don't compete to be heard. This process involves several factors, and I actually believe this is the most difficult part of the process, many call this phase post production. I want to utilize as much of the spectrum of sound I have available to me taking into consideration volume, frequency conflicts, panning (especially in my drum parts), harmony balance, the list goes on. This forum has many many great tutorials for post production as does youtube and most other outlets online for production assistance. I think there are added benfits in this phase to have others listen to your track, listen to it on multiple sound systems ( Mp3 player, car stereo, studio sound if it is available to you, live performance setup if it is available to you, as many different envirnments as you can. ) This will allow you to make the small tweaks needed to optimize the balance of your track in all listening environments!
All the techical talk aside, the process is what you make it. Once you have really learned to listen to yourself for basic ideas the rest is really what you make it. A simple line stuck in your head that comes strictly from you or is inspired by other work is the foundation for your musical endeavor! I hope this helps some, I know I didn't speak a lot about the specifics of DAWs and VSTs and this and that. It didn't seem to me that those things are exactly what you are asking for though. I think you might just be looking for insight into other composers' creative steps. The last thing you say is "how do you estimate how much work is remaining to complete a track?" In my opinion, a track is rarely "finished". You can always add nuance, development, additional voices etc. It's all up to you as a composer! I hope this helps! Send me anything you would like to and I'll listen and give constructive feedback! :mayonesa: