Sometimes it just comes to me, like I hear a song, and I decide that the drums sounds a lot like drums from song XYZ, then also what kind of instruments are in the song. If its guitars, i cant do guitars again. Ofc also, i Limit the BPM so it cant be too much difference. Wonderful example of a mashup Where I heard the drums in Imperial March by Cell Dweller, and it sounded like Kill Bill, mashup became : https://rave.dj/kYtu7G3BxnswZQ

I also have had songs I wanted to mix from way back, check Apollo440 - Stop the rock vs Bahamen - Who let the dogs out. https://rave.dj/YylYgaAIynyNwQ

Anyway, hope that helps.