Having witnessed Hibs defeat Aberdeen the day before in the Scottish Cup semi-final, Hearts travelled to Hampden Park as underdogs against the Parkhead side.
Plans, however, were in place as soon as the teams were pitted against each other in the last four and the strategy devised by the Portuguese boss worked a treat.
Hearts took a second half lead through Rudi Skacel and, despite conceding a late equaliser, substitute Craig Beattie netted a famous winner from the penalty spot.
McGowan told Open Goal: "As soon as the semi-final came out Paulo Sergio worked on, not a formation, but he was so rigid and we knew what we were going to do that the longer the game went with that game plan the more belief we got.
"He played young (Scott) Robinson on the right. So we played a 4-5-1. He didn't hang him out to dry as such but said 45 minutes of buzzing around, being as busy as you can and then we will probably bring on Beattie and change the formation, they wouldn't be expecting that and we'd nick a goal there.
"The whole plan was to get to half-time 0-0 and then we can bring players on to change the game and try to do it. It actually worked perfect. Beattie came on at half-time and Rudi scored straight away.
"We had a sense that season that on big occasions we had players who could score goals and win us the game."
Sergio's preparations also included practising penalties in the build up to the game due to issues from the spot that campaign.
Having witnessed him in training, McGowan was certain Beattie would score when referee Euan Norris awarded the penalty.
He said: "We were struggling for penalties, we had missed a few. Hamill used to take them, he got injured. Grainger took a couple, missed a couple. Skacel didn't like penalties for some reason.
"The day before we worked on penalties. Sergio made us take seven penalties in a row, we had to walk from the halfway line. He wasn't playing for a draw but we would have been happy with a draw.
"Beats was really good, straight down the middle. So when we got the penalty I was buzzing thinking 'I fancy him'. Fraser Forster in goals (for Celtic), he's going to have to dive, so as long as he gets enough height on it he's going to score. I was relatively confident he was going to score."
And, of course, the famous celebration which ensued.
"A few boys were on yellow cards and Paulo Sergio said don't get a stupid yellow for celebrating or dissent and you end up missing the final," revealed McGowan.
"Darren Barr and a few of the boys were on yellow cards. So when Beattie scores he goes off the pitch and Darren Barr is on a yellow and he's chasing Beats then it's almost as if he sees the line and puts the brakes on, stops and doesn't go.
"I'm thinking I'm not on a yellow, I'm going as well and just chased Beattie around. "