To isolate a specific instrument, open SAM Audio Online, choose the custom prompt option, and describe the sound you want to isolate. Prompts such as "saxophone," "synthesizer," or "electric guitar" create a matching stem and a stem containing the rest of the mix.
Sounds that overlap heavily can bleed between stems. For a difficult section, open the track in Studio and mark the time span where the target sound is easiest to hear. You can start from SAM Audio Online or the Universal Stem Splitter.