I agree with most of what TopoMorto said in his answer and I wish to only give you some more information, and ideas, and some of my own experience as a tip.

In most cases the melody is the main thing in music. I guess you always have some tunes playing in your head! I know I do! Certainly that's where I start once I have a melody. I take the piano or guitar if I have them and start playing the melody, finding those notes in my head and the intervals. Or I just start writing those notes in Guitar Pro if I have no instrument. If my melody is interesting for me I leave it as is else I start thinking about chaning some of it in some places to make it more interesting.

After I've finished the melody (it could be the lyrics by the way) i start finding good and interesting chords. I play them in different ways in different variations. On a live instrument making up a melody and harmony usually goes together. Sometimes I start with the rhythm when I'm unsure at what I wish to have in result.

Scoring a piece isn't composing... you should already have the piece you wish to score. And it's usually simple once you know what each instrument should play. Bassically in scoring the most difficult part is knowing how to write the notes and the elements on the claves and the staff. Knowing how to show the player what you wish him to play. I mean you need not only to write the correct notes that you play but also the stresses you make and the effects such as tremolo and vibrato and fade in/fade out effects as well as many many others.

I start with a guitar or piano (those are my main isntruments). I start singing or humming a melody and just playing (improvising) and searching for an interesting chord progression if I have no idea in my head. If I have one (maybe a lyrics) I start singing the melody or the lyrics and start looking for a harmony all at the same time! Once I've found the basic harmony and the melody is okay. I start playing in different styles and adding extra chords and notes. yeh, I start by playing mostly simple chords and no arpeggios or whatever. When I have some good interesting chords or arpeggios I usually record that. After I've recorded I play it and listen... and continue improvising in my head. If I feel I've spotted an interesting moment.. I stop and play that part with the new idea. After I've made the piece look good for melody and one instrument harmony. I start playing a second instrument to the music and create an arrangement. I tap out a rhythm and think of a bass line and etc. During the whole process I might several times change my main instrument if somewhere some note is disturbing or doesn't sound right with other instruments. I often do all that for a very long time. Once I have most of the music written I start thinking about a solo and effect If i want. Usually a solo takes up a lot of time if I wish to make it really interesting and not just a simple improvisation. I might even change the harmony is some places according to a solo I'm building. In the end, finally, i get a nice good arrangement ready for my whole band to play. Most of my arrangements are in my storage in gpx Guitar Pro 6 format.

I agree with most of what TopoMorto said in his answer and I wish only to give you some more information, and ideas, and some of my own experience as a tip.

Almost always the melody is the vital part of any music. I guess you always have some tunes playing in your head! I know I do! Certainly that's where I start once I have a melody. I take the piano or guitar if I have them at hand and start playing the melody, finding those notes in my head and the intervals. Or I just start writing those notes in Guitar Pro if I have no instrument by my side. If my melody suits me then I leave it as it is, otherwise I start thinking about changing some of it in some places to make it more interesting.

After I've finished the melody (it could be how the lyrics should sound by the way) I start finding good and interesting chords. I play them in different ways in different variations. On a live instrument making up a melody and harmony usually goes together. Sometimes I start with the rhythm when I'm unsure what I wish to have in the end.

Scoring a piece isn't composing... you should already have the piece you wish to score. And it's usually simple once you know what each instrument should play. Basically in scoring the most difficult part is knowing how to write the notes and the elements on the claves and the staff. Knowing how to show the player what you want him to play. I mean you need not only to write the correct notes that you play but also the stresses you make and the effects such as tremolo and vibrato and fade in/fade out effects as well as many many others.

I start with a guitar or piano (those are my main isntruments). I start singing or humming a melody and just playing (improvising) and searching for an interesting chord progression if I have no idea in my head. If I have one (maybe a lyrics) I start singing the melody or the lyrics and start looking for a harmony all at the same time! Once I've found the basic harmony and the melody is okay. I start playing in different styles and adding extra chords and notes. I start by playing mostly simple chords and no arpeggios or whatever. When I have some good interesting chords I usually record that. After I've recorded I play it and listen... and continue improvising in my head. If I feel I've spotted an interesting moment.. I stop and play that part with the new idea. After I've made the piece look good for melody and one instrument harmony. I start playing a second instrument to the music and create an arrangement. I tap out a rhythm and think of a bass line and etc. During the whole process I might change my main instrument several times if somewhere some note is disturbing or doesn't sound right with other instruments. I often do all that for a very long time. Once I have most of the music written I start thinking about a solo and effect If i want. Usually a solo takes up a lot of time if I wish to make it really interesting and not just a simple improvisation. I might even change the harmony is some places according to a solo I'm building. In the end, finally, I get a nice good arrangement ready for my whole band to play. Most of my arrangements are in my storage in gp Guitar Pro 7 format.