Well i used audacity for it. First thing i did is find out the BPM and key of both songs and then use the Tempo effect (idr the exact name) to SPEED UP one of the songs to match the other (avoid slowing down with tempo, it will introduce unpleasant glitchy effects). Ideally the BPM and key should be close otherwise youll have to heavily distort one to fit, which doesnt sound good. I dont have any experience with changing key so cant say anything there. After that you have a basic mashup. I usually also edit the songs a bit to maybe cut out/prolong some sections to better match. Usually just try to cut it right on the beat (audacity has a bpm snap option iirc). Actually cutting up the songs is the really hard part, though it depends on the songs. Also i find for mashups its best to use simpler sounding songs. If both songs have a lot going on youll just end up with an audio sludge. Thats why Waters Of Nazareth has so many mashups imo, its basically just the bass layer of a song that is ripe for adding on top of (and also the song is just iconic and good too of course) Luftrausers theme is also somewhat alright for it imo, and most songs in brutal orchestra are pretty simple aswell. I cant give much other advice as this whole mashup business is basically just based on vibes and your own taste, so good luck!

Oh and i used the Tempo change to avoid changing the pitch of the songs. You can use Change Pitch to correct the songs key but only if its close to the other song, changing it too much will distort the song too much.

@cheesebaron7533 Thank you so much for the tips! I wondered if audacity was good for music but didn't really know until you told me. Also before this I may or may not have been trying to kinda hack it by just manipulating audio files in a video editor, but that didn't have the tools I needed and was altogether kinda a terrible idea. I promise I will make something befitting the wonderful game Luftrausers is!

I didnt make the art myself but as far as i understand my friend took various stock images from google and edited them in paint.net

Yes.

There's even the plane behind him

_BOOOOO!_

Really i just thought they sounded similiar lol. And then the creator confirmed that trigger fingers theme was inspired by the luftrausers theme so thats neat.

Bought Luftrausers because of this

Dont really know exactly but the first playable version was uploaded to itch.io on june 29 2019 and the last update i published was on october 11 2019. So around 5 months? (Sorry for answering this late)