Mastering the master… help wanted!

I think the reason it doesnt quite sound right is the songs are in different keys.
They sound almost compatible with each other but maybe not 100%

When doing a master for a mashup there shouldnt be much work to do at all. The instrumental you are using is
already mastered so just running the acapella through a compressor to iron out the peaks should leave you with
a perfectly playable track.

A pro mastering engineer will vary their plugin chain depending on the tasks he / she needs to do to get it sounding as big & clean as possible. However, as a general rule, most mastering chains go as follows:

Some form of stereo tools (ALWAYS best to fix this in the mix, as it’s your own music you should have no need of this at mastering stage but sometimes I get sent stuff which needs for example everything under 100hz mono’d)
Saturation
Hi quality EQ
High quality compressor
Limiter

As you should be using absolutely the best quality plugins you can find for your mastering, it’s often a great idea to import the wav into a fresh empty project, or you’re unlikely to be able to hear what you’re doing due to processor stuttering unless your pc is a beast! Also make sure when exporting your track to do it at 32bit floating point mode, this means that you essentially get a little bit of extra headroom. Anything lower than 32bit floating point, then if your track clips at all, turning it down at the mastering stage will merely be turning down a clipped signal; whereas if you export at 32bit floating point, the extra headroom means you won’t get nasty clipping-based distortion (or simple allow the compressor to do its thing).

Hope that’s of some help

ok cool thanks