Code:
    
main      dps
     #show Expolsive Shot
     /click MultiBarRightButton1
     /click MultiBarRightButton2
     /click MultiBarRightButton3
     /click MultiBarRightButton4
     /click MultiBarRightButton5
button      1
     /castsequence reset=combat Hunter's Mark, null
button      2
     /castsequence reset=combat explosive shot, null
button      3
     /castsequence reset=combat Black Arrow ,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,
button      4
     /castsequence reset=combat Serpent Sting,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,
button      5
     /castsequence Steady shot,
     /cast Explosive shot
This should start with an explosive shot, place the dots, use steady shot as a filler and fire explosive shot when it's up. (You could use the !Autoshot trick mentioned in olipcs link to get the timings a bit better)