The final step before sharing the song with anyone, is mastering. As with mixing, this is its own profession, where professional mastering engineers apply the final touch that makes the song stick together, consistent and loud. Yet again I had no budget for this so I had to do it myself. When I started out, I had no idea of how to this, however there are plenty of online resources to read, watch and learn from.
For my songs a applied just a few effects and the end result sounds much better then the unmastered mix. However a professional would get it better still…This is how I did it, first I apply a EQ cutting all frequences lower than 30Hz. This frequency range is not audible but could add unwanted distortion, so it is better to get rid of it. Now in mastering, I work with the master track, not with the individual channels.
Second I add a compressor. Long attack and release times 60-100ms and a low ratio 1.5, this will help glue everything together. I use the standard builtin Reaper compressor ReComp for this.
Then I sometimes add another EQ, if there are particular things that I feel needs adjusting. But typically that is handeled in the mix instead.
The next step is to add a little bit of stereoenhancing. For frequency exceeding 500Hz I ad a little bit of stereo enhancer in order to make the sound a bit more interesting.
The last step is to make the track loud. Here I apply a MasterBus limiter. The limiter is basically a compressor with high ratio and will ensure that I do not exceed 0dB and get clipping.
Done, that is it, then I listen to the end result on my headphones, in the car, the other car and another couple of headphones and speakers. If it sounds good on all units, then it is good to go, otherwise go back and iterate!