http://www.basstabarchive.com/howto.html#what is tab
unless the song is really simple, writing tabs requires a bit of working knowledge of how to play the bass. Since a given note can be played on more than one string and has different positions on each string, tabs also tell you which string/fret to use.
Each string has a name, based on the note it plays when you pluck an open string. On a 4 string bass it the strings are a fourth apart, with the lowest string on the bottom in tab, like this.
G
D
A
E
When you write a note, you are telling them which fret to hold down on which string. a 0 means play an open string (no frets pressed down). A 4 string bass' lowest note is an E. Tab to play it looks like this
G
D
A
E 0
Each fret will take the string a semitone higher. Low E is a semitone higher than E so its 1 fret up the string, like so:
G
D
A
E 1
heres how I would write the first bar of twinkle twinkle little star in G
G ----------------
D --------2-2----
A ----5-5-----5--
E 3-3------------
To be redundant, the first two notes are G, which is 3 semitones up from low E. The second two are D, which is 5 semitones up from A, and so on.
altho this would work as well
G ----------------
D ----0-0-2-2-0--
A ---------------
E 3-3------------
as would this
G ----------------
D ---------------
A ----5-5-7-7-5--
E 3-3------------
or this
G ---------------------
D ----------------------
A -----------------------
E 3-3-10-10-12-12-10--
altho its harder to play it like that since the player only has 4 fingers, and the tab quickly jumps 7 frets. Moving up strings is always faster than sliding yout hand up the neck. This is why in general you need to play bass to write tab - knowing when to slide up and down the neck is something you only get through playing experience.
But in your case it sounds like you have no specific part, so you don't need much range to show the bass player what notes to play. Don't use a fret number higher than 4 and the hand position won't be a problem. You have about 1.5 octaves to work with, but its enough.
rhythmic info is trickier in tab, but you can at least write bars the same way as in normal sheet music. You will essentially only have to write one note for each bar.
Bass players don't play chords all that much in most music, so don't worry about that.
