I have included notes where appropriate.
The second is the 'a' sound in words like 'map' and 'hand.'
Flap not thy hand like that.
So which is which? To most folks, the two sounds seem alike in their native dialect. There is a general rule, but it's pretty complicated: make the first change (as in 'master'), when the 'a' comes before 'f,' 's,' 'th,' and 'n,' unless the 'n' is followed by a 'd,' as in 'hand.' See? I told you it was complicated. To make things worse, there are a few exceptions, such as 'command.' This rule is only designed to get you through until your ear gets used to hearing these sounds.
Notice the difference between 'do' and 'due.'
Finally, here's a little verse to practice with. I recommend using this poem to get into dialect before rehearsal or performance.
And there thou hast it. All the changes that thou dost need to speak the Lower Class Dialect. I wish thee much joy of it!