3VID3NC3 has a good technique. Just match the metronome with the original song until you get it as close as possible. Import your A cappella and start positioning it to get it nice and tight. You can even just match the metronome to the A cappella itself. I like to map out the tempo to the hook of the song, so I will basically loop the hook and play with the tempo until I get it close, say about 90-95%. Doberman makes a good point about the A capellas, there will be space on them, you need to really tweak the positioning to get it right, in most cases where you know the exact tempo of the song before going in, you can't just snap the track and have it on beat, you'll notice you really have to tweak it, even if you have the tempo, this is due to the fact that it's has air and space and you have to position it just on the right spot. Doesn't have to be exact, you just have to get it on beat, then you can fine tune the tempo later. You can do Beat Mapping in most software DAW's. After you have the A capella on beat, pick your drums and start laying down your drums immediately. Again, you really just need to loop the hook and build the song on the hook. Once you get your drums sounding right you can move on to the next step which would be for me, finding the right synth and playing with melodies, finding something catchy, that stands out and goes with the track as a whole. I like to start with a synth sound that I can really get creative with and build on top of.