The following may give you some ideas on how to flesh out that song. Long story, but, best to start at the beginning. Let's first get the song under control then we can work on the bass line.
There are several step we need to take into account when writing a song. It really does not matter which step we take first, however, before we finish we will have to tie all the steps together. I'm Country so I normally start with the story, The story is a big thing in Country. That means I start with the lyrics. Usually that will be a four line verse, some time they rhyme some time they do not. I'll end up with three verses and one chorus - in the first draft.
I pick a key based upon who will be singing the song. I sing in D, Shirley would like A, Tom would prefer C or G. My point if you pick a key/scale right off the bat it sure makes everything else much easier.
I'll need a chord progression to move the story along in the verse, so I start with a dirt simple cookie cutter chord progressions. One that has written thousands of songs before. I normally settle on I-IV-V7-I. Start the verse with the I chord and move to the IV chord near the end of the first line of the verse. The IV chord continues into the second line. Near the end of the second line the V7 chord will appear and then quickly resolve to the I tonic chord. The 3rd and 4th line of the verse will follow this same format. Remember it's a first draft. I like having a V-I cadence for every two lines of the verse. Use this same chord progression for all three verses, the chorus could also use it or you may want something different. Up to you.
I next move the chord progression's chords a little to the left or right to match up with the lyric words, i.e. get the chords to flow with the story. Remember I just kinda threw (penciled) them in "close to the end of the first line". Time to close in on the actual chord change word.
After the lyrics and the chords are flowing together I then write the melody. Go to the keyboard for this. The melody line and the chord line should share like notes. Easy way to accomplish this is to use the chord's pentatonic notes. That will give you three like chord tones for harmony and two passing notes for interest. More than enough melody notes for a first draft. Remember you will be using at least three chords.
Take it in phrases - one melody note for each lyric word, two syllable words get two melody notes. The chord used in the song dictate the melody notes I'll be using. This way my melody line and my chord line will always harmonize. Recite the lyrics and pick the pentatonic notes that best fit over the lyric words. Mary had a little lamb in C, yep the notes of the C major pentatonic worked fine. C major pentatonic = C, D, E, G, A and the notes of Mary had a little lamb, in C major, are; E-D-C-D-E-E-E-D-D-D-E-G-G.
That gets a lead sheet. Treble clef and chord names - no bass clef. Time to work up your bass line. Mine will usually come from the chord tones. Roots, fives, eights and the correct 3 and 7 will write a pretty good bass line. For example Cmaj7 chord could have a bass line of R-3-5-7 and the G7 chord could be R-3-5-b7. Am chord could be R-b3-5-8. Usually lot of roots ( R-R-R-R) and root/fives (R-5-R-5) flow into a groove much easier than a bass line using all the chord tones. Of course IMHO. That will give you a bare bone dirt simple first draft.
That took a long way around to get to how to write a bass line for that song. I hope it let you see how everything is connected.
Does not matter if you start with the lyrics or the melody -
when you finish all of the above should fall into place and fit together. For example:
- Lyrics dictate verse format.
- Chords move the story along through the I (rest), IV (tension), V7 (climax) and I (resolve) back to rest journey the verse should take.
- Chords also harmonize the melody by sharing like notes at the same time during the song.
- So at this point we have the melody line and the chord line harmonizing. Our bass line should 1) lay down the beat and 2) accent the harmony while accompaniment is our task. Roots, fives, eights along with the correct 3 and 7 will play a lot of bass. Then when soloing we accent the tune, "let the melody be your guide", either with scale notes or chord tones - your choice.
- If everything is in key and in harmony things work out for the best.
Have fun.
Need help composing bassline for original song. 
