CO2 is removed
Remaining H and H from neighboring Thiol are removed.
Molecule usually has a ketone on the alpha carbon, next to the carboxylic acid which is on the first carbon.
A molecule is decarboxylated CO2 ( O=C-O ) is removed, leaving just the H where the Carboxylic acid used to be.
Then NAD+ ( NADH + H+ ) come a long and take the H from the molecule that had the carboxylic acid ( now removed ) and a neighboring Thiol ( HS... ) so that the S from the Thiol and the C from the other molecule bond to form a thioester.
The molecule will usualy have a ketone on the alpha carbon, next to the carboxylic acid group which is on the first carbon.