it depends for me, on how creative i actually am at the moment lol....the chord progression is usually the easy part, tho sometimes it can challenge me.....but usually i'll lay down the progression....and since i know what scale i'm using, and what keys are in that scale, i'll jam around until i find a melody or bassline...depends which instrument i'm on at the moment....and just keep on building.. theres really not 1 set method, as it really does vary from track to track
google and youtube are your friends.....theres tons of websites and video tutorials out there that explain, and demonstrate exactly what you're asking..........And if you go into the Tutorial section on this website's messageboard, Bandcoach recently posted a thread about how to improve melodies...so you wana check that out too, cuz he knows his shit