i need input about dpo

ok so do u count the days after ur peak or a day or 2 after u have ur peak ovulation???? i had my peak nov 30th?!?!... used mucinex and preseed that day n day after hope for a bfp, but not sure when to start counting dpo...i have been trying for 3 yrs n i have 2 boys n i lost a boy at 24 weeks pregnant... not sure y i havent got pregnant... i jus want to give my husband a baby bc the 2 i have r not his and the boy i lost was his so i feel like im letting him down...