To pull everything together in an example:
If you have FBSS (21% Haste) and Silver Chitin Hand Wraps (22% Haste) Equipped, your final WORN HASTE is 22%.
If you have the Celerity Buff Spell Haste (50% Haste) and Villia's Chorus of Celerity Bard Song Spell Haste (45%), your final SPELL HASTE is 50%.
If you have Composition of Ervaj Bard Song Spell Haste (10%), your final V2 SPELL HASTE is 10%
So your total haste would be WORN HASTE (22%) + SPELL HASTE (50%) + V2 SPELL HASTE (10%) = 82% total haste. This assumes you are not level capped on haste. Level caps for haste can be found here:
https://wiki.project1999.com/Haste_Guide