Like is said, practice makes it easier.
My method varies. Most of the time I start with my bass in hand as I'll often play something similar to the vocals or it'll be riff based. I set a simple structure (such as intro, Verse, Chorus, Verse, Chorus) & work on it from there, adding bits as the song starts to take shape. A good example is this: I started off with a melodic bass line, & let my singer hear it. He said "I've got some lyrics that I haven't managed to write a song for as I can't get them to fit, let's try them". He sings, they fit. I then amended things to accommodate a chorus & then it started to grow legs. It got a key change for an instrumental break & that completed it nicely to be Intro, Verse, Short Break, Verse, Chorus, Instrumental Break, Verse, Chorus x2, Outro. took about 6 months to write from start to finish (most songs are much quicker) & we called it Little Disappointments (you can hear the original bit of bassline on one of my reviews in effects reviews if you're interested).
Another song I started with trying to get a sine like sound out of my bass & came up with a 3 chord verse (C, E & A), then made a wee melody on the guitar which ended up being the vocal line & replaced the bass with guitar & it's now played with just 2 guitars & a flute. It's called Johnny & Sid & started of as a rock song about punk but we now cover it as a folk song.
Other methods I use are either starting with guitar for a chord structure or riff or using music software & working on the mac.