mix master vocals into a rap beatOnce the vocal is put at a proper place the process of “matching” it to the mix is started. The idea of it's mixing services is very simple - to make it sound like it belongs to the instrumental.
Compression
Let’s study it with the help of an example. Your backing track is over compressed and the vocal obtains a rather high dynamic range. As a result the vocal will either sound as if it’s buried by the beat or will so loud that it will seem it’s hanging over the mix. No one from the mentioned instancess is too good. To reduce such unfavorable moments try to compress vocals till their density reaches that of the instrumental. You will need a special carefulness and probably co-work of several compressors to reduce the negative effects of the excessive compression. If you operate with utter carefulness you will eventually be able to compress vocals avoiding possible adverse consequences. But in this case you will most likely have to reach a compromise.
mixing mastering vocals to rap instrumentalFrequency-Response Characteristic
An instrumental can be either very bright or dark. You will need to fit the equalization of the vocals for matching the track. A bright vocal will sound poorly on a dark instrumental. If the beat slightly saturated then you should use saturation to the vocals. Anyway be reasonable and don’t kick over the traces. 

You can also be interested to learn more about how to mix vocals into an instrumental track.