Assume we're playing in the key of "Bb" for all the following progressions. Also, I'm going to use arabic numbers instead of roman numerals for the progressions.
The diagram will be written as such:
----------
----------
----------
----------
...representing each string on the bass, the bottom line being the E (low) string. Fret positions will be written as arabic numbers on the line that you should play and in the order that you should play them left to right, as such:
-----------
-----------
---8-------
-8---6----
...means that you play C, the 8th fret on the E string, F the 8th fret on the A string, Bb the 6th fret on the E string, in that order. With that, here are the major progressions:
1-4-5:
-----------
-----------
---6--8---
-6--------
(Bb, Db, F)
2-5-1:
-----------
-----------
---8-------
-8---6----
(C, F, Bb)
7-3-6:
-----------
------5----
---5------
-5--------
(A, D, G)
3-6-2:
-----------
----5------
-5--------
------8---
(D, G, C)
5-1-4:
-----------
-----------
-8---6----
---6-------
(F, Bb, Db)
The following diagram shows where each part of a progression (or note of the Bb scale) is located on the fretboard for the key of Bb:
||(1st)----------------(5th)----------------------------------(12th)-----------
||---+-7-+-1-+---+-2-+---+-3-+-4-+---+-5-+---+-6-+---+-7-+
||-4-+---+-5-+---+-6-+---+-7-+-1-+---+-2-+---+-3-+-4-+---+
||-1-+---+-2-+---+-3-+-4-+---+-5-+---+-6-+---+-7-+-1-+---+
||-5-+---+-6-+---+-7-+-1-+---+-2-+---+-3-+-4-+---+-5-+---+
NOTE: Again, 1-7 denotes the keys of the Bb scale or the part of a progression. Also, this patter is good for all keys. All you have to do is rotate the bold part of each line left or right together. This part repeats because the 12th fret is the same as playing an open string one octave higher.
I'm sure this will help anyone get started with the bass guitar, and begin to understand the concept behind what's going on. This doesn't show you how to do improv, but keep the basic bass line of the progressions.