Watkins had only played 20 minutes in five games in Germany before coming off the bench to score a stoppage-time winner to beat the Netherlands 2-1 in England's semi-final on Wednesday.
The Aston Villa striker's only previous goal in a competitive game at international level had come against San Marino in World Cup qualifying three years ago.
"It's been an unbelievable feeling to be honest," Watkins told reporters on Friday of his stunning low strike against the Dutch.
"I knew when I was going onto the pitch that I was going to score, you know, I manifested it. I had worked very hard to get to that point, so it's been an amazing feeling, but I'm not shocked."
Watkins' only previous appearance at the tournament had been as a late substitute in a 1-1 draw against Denmark in the group stages.
He had to be patient for his opportunity despite England's struggles to get going in attack.
Gareth Southgate's men have only won two of their six matches at Euro 2024 in 90 minutes and have needed late goals and comebacks in each of the last three rounds to get past Slovakia, Switzerland and the Netherlands.
However, Watkins is hopeful England are coming to the boil in time to face a Spanish side that have been a class apart in the competition so far.
"Obviously the early stages, the group stages, we weren't playing our best football. It doesn't take a genius to work that one out. Everyone knew, even in the base camp," added Watkins.
"The main thing is, we weren't losing games, so we were unbeaten. Then progressively, as the tournament's gone further on, we're playing better football, we're controlling games.
"Even if we've gone a goal behind, we don't crumble. We've come back and shown the togetherness, that attitude, that everyone's there as a team to work hard and pull together."
Spain's serene progress to their first major final since a golden generation won three tournaments in a row between 2008 and 2012 is even more impressive given they have had to come through the tougher side of the draw.
Luis de la Fuente's men have beaten Croatia, Italy, Germany and France en route to Berlin.
Young wingers Lamine Yamal and Nico Williams have caught the eye and Watkins pinpointed them as the main threat to England's hopes of becoming European champions for the first time.
"I feel like they have been probably the best team of the competition so far," he said.
"Obviously, the wingers for Spain are very dynamic, young, full of confidence and direct, but also across our team we have goals from everywhere, world-class talent.
"It's going to be a very exciting match. I can't wait to watch it and be a part of it and hopefully we can get the job done."
