I usually start off by trying to find the tempo/timing, so I lay down some hi-hats & snare/clap to get an idea like a metronome, then usually a bassline or other non-bass melody. There's really no proper way to do it though, as sometimes I start off with the main melody or bassline as well, then lay the drums after.
But like Mr. Samos said, I usually do the same, I usually start off with a Piano patch or Guitar patch when trying to come up with melodies. Sounds like he uses a REAL guitar to come up with melodies, if you can do that, do that as well.
There's really no formula to follow. It all depends on your style and what's more effecient for you. Like one of my homies loves to pretty much always start off by coming up with basslines, so that's his thing. I personally do it in different ways. So if my homie was to have a formula for his shit, it would be bassline 1st, perhaps drums next, etc...etc...
I guess you can try to come up with a formula of sounds to follow by. So you can have beats that have simliar sounds in them. Might sound repetitive, but I think it's a way to try to establish your OWN sound and identity. As I personally strive to use the piano, guitar, E.Bass, strings most of the time in my beats. I guess it's the same thing as a rock band using their favorite guitars, favorite drum sets, ... Like a mariachi group using their trumpets, guitars, violins, accordians, etc... The thing is though with Rap, your sounds can be so diverse, these days they range from piano to techno sounds, so who the fuck knows. So I guess Rap doesn't really have any correct sounds to follow by, I personally just like using a lot of piano and guitar especially so that's kind of the formula I've been sticking to.