I don't claim to be a stellar songwriter, but ArcaneSoul was asking me about writing melody and the response got huge so I decided to put it here.
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1. Write a good chord progression. you already know how to do that probably better than me heh.
2. Choose a good instrument. An instrument that will sound good in almost any situation. the sound of an instrument really makes a difference to how the melody ends up sounding, and some sounds are more versatile than others. my favorite starter instrument are reverbed saws.
after I do this, i kinda just listen to the chord prog on repeat for a while while i use the computer keyboard to mess around on top of it and get a feel for the progression. if i'm lucky, i'll find a neat idea instantly and write it down.
3. Move to the piano roll and improvise. one neat trick i've found is to write like 2 notes, then play them and see where my mind naturally thinks the melody should go. Like if the first chord of the prog is C major, i could write C-E as a third and then play it, and then i might imagine hitting a higher note and then falling down or something. this is as close as you can get to improvisation without a midi keyboard, heh. my best melodies have come from my mind trying to fill in the start of a melody. it's a really helpful technique.
normally when i do the piano roll i need to do a few bars of melody before i really get something interesting. i think it just takes time for the progression to sink in. you can't be interrupted at this point. you get into melody writing flow after like 10-20 minutes of hearing the progression and improvising over it, and distracting yourself with anything else pretty much returns you to the initial point of writing boring melodies again.
DONT GO BACK AND EDIT YOUR MELODY WHILE YOU ARE IMPROVISING. don't worry about how it sounds crap. criticizing it now or even changing it too much is a really easy way to fall out of the flow.
4. Fill in the sketch. after i do this and im happy, it's like i've finished with a sketch. the core melodic idea is there, but now i have to flesh it out. this usually entails adding bends, filling in gaps with softer notes, or even adding arpeggios. The reason I do this afterwards is because you dont have to be in flow in order to fill in.
5. Make your idea stand out.the idea of tweaking is to get at that core idea you had while improvising and make it really stand out. most of the time when you write a melody the core idea will be good but it'll be hidden by extraneous notes and irregular tempos. if you aren't sure if you have a good core idea, dont be afraid of going back to improvise more. generally, the more the better.
this 'filling in' period can take a melody from good to great, but almost never from bad to good. if you're not satisfied with your melody at all at this point, don't be afraid to write a new one. it's really easy to get attached to stuff that you write, even if you know that it isn't spectacular. i know this more than anyone. lol.
but what if it sounds just a bit off, like you think it could be improved but you dont know how? here's a tip: step away from the computer, take a 5 minute break, and try to remember what your melody sounded like. if you can remember it perfectly, it's good. if you can't remember it at all... you may want to start over. BUT. if you can't remember it, but you can remember something close... congratulations, you just came up with a better melody than what you have. the best melodies are simple and memorable, and your mind just made your melody simpler when you weren't even thinking about it.
(and note, this isn't a hard and fast rule. If your song is very serious, or really emotional, it's possible that your melody isn't memorable at all, but none less powerful. Radiohead comes to mind. )
6. THE MOST IMPORTANT PART.
Every artist is different. Every artist's creative process is different. Don't take this as the One True Path to great melodies or something like that, obviously. Try it out to see if it helps, and feel free to discard anything that doesn't work for you. Except that part about "tons of practice". lol :D