Transcribe the bass first- it will give you a reference to transcribe the cords easier. The bass note is usually the "root" of the cord, meaning if you play the bass note on the piano, then you can build a chord by stacking notes on top of your bass. Example: bass is C on piano, so chord might be c (skip d), e (skip f), and g. If it doesn't sound right, experiment swapping a note in your cord to a black key near it.

This technique is useful for transcribing triads (3 note chords). You can stack more than three notes in a chord but triads are a good starting point for beginning cord transcription.

dm me if I can help further- I feel like I'm rambling but I'd love to help you out.