For me i follow this procedure almost every time.
1. Listen to the song and get to know it first.
2. play along with the song to get a feel for the bass part.
3. i use anything i can find on line from full music sheet to bass tabs to basic song chords.
4. Break the song into sections ie Intro, verse, chorus, mid sections, and ending (end might change with your band as most endings fade)
5. Write out the bass part in whatever manner suits you. I'll write the song out in individual bars with every note and spacing as i think it should be. At this stage its just scribbled notes. You can write it out as very simple notes. whatever suits you best.
6. Learn the song from your notes while playing along with the song.
7. Once i've learned the song and can play it with ease i take my scribbled notes and write them out properly on an A4 paper trying to keep it to one page if possible. This allows me to tidy things up for rehearsals and i can read easily if required. (see a pic of my notes below)
Doing it this way means i learn the song by playing along with it and i also remember what i've written out on the A4. It seems to work for me plus i always have the music written out and filed for any future bands.
Dave