This page might be of interest. She teaches some tricks and advices on how to play Swedish folk music and also teaches some tunes.
https://www.youtube.com/c/EmelieWaldken/videos

This video talks a little bit about why Swedish folk music is not described well into sheet music and the tradition is to learn by ear.
https://www.youtube.com/watch?v=RmkNer0jWVA&list=PLNj2qBmoNX5jR0y3YrBfOMZOkeATtKxVZ&index=6

Most people learn or get started from a more experienced player that know the tradition and how it should sound and there are many people that play in “Spelmanslag” (lot of performances on YT ).
Nyckelharpa players often play mixed together with fiddlers.

If you are on your own one way to learn could be to:
Try and find a good representation of the tune to listen to with the right feel (either from CD or YouTube or other).

There are so many different versions out there sounding very different and the tricky bit is to find a version that has the feel and sound one is looking for.

“Spelmanslag” often play a very good representation of the traditional tunes but it can be a very massive sound and a bit difficult to pick out the tune from it.
It would probably be more easy if one could find a version of the tune from a good solo fiddler or Nyckelharpa player https://www.youtube.com/watch?v=fnMRqhWFyyg

Play the tune repeatedly on and on for several days to get the feel of the song into the head and sleep on it (it matures during night).
Then try to pick out the notes by ear little bit at a time (as support one can try write down the letters for the notes in sequence on a paper only the letters or names or fingering)…
.. or try to find a notation with the basic melody in the same key just to try to find most of the notes in the basic melody. Then try to adjust it bit by bit to match the audio version of the song.

There are some good software available that can slow down music(audio and video and keep the pitch with very good quality.
I use a software from https://www.seventhstring.com/ called “Transcribe” (It might be a bit tricky to install).
Thera are other slow downer software around but I find that Transcribe has far most the best “High Fidelity” which makes it so much easier to hear the notes as well as good video slow down.

Here is link to a lot of music for Swedish Folk music http://www.folkwiki.se/

BIAB 2026, Studio Pro 8, Song Master Pro, Win11 Home. i7-9700K CPU, 32GB, ESI MAYA44eX, ZOOM UAC-2, Guitar Pro 8, Transcribe, (EZKeys2, EZD3, SD3, EZBass, EZMix3)
Amateur: fiddle, guitar, vocal, beginner on bass.