"I read about a child who had done something similar," he said.
"Then I had a discussion with my brother about whether it was possible to do something like this. Then I just cracked on with it.
"I put the video on my Facebook page and some of my friends saw it and said I should put it up on YouTube.
"It was well worth doing," he said, admitting he will "probably do the final if we win."
But, perhaps unsuprisingly, he confessed: "I am quite pleased to get it finished.
A season ticket holder from Sully, Vale of Glamorgan, he studied footage of the game to make sure the Lego characters' positions were accurate.
"I just tipped the box out on the floor and looked through it to find the right colours," he said.
The film recreates Joe Ledley's semi-final winner against Barnsley
He used a normal digital camera to take about 1,000 stills, altering the positions of the Lego men slightly each time. Each second of the film has five or six stills, and it was created by being put through a movie-making device on his computer.
He moved the ball by attaching a piece of cotton to it. This cotton was wrapped around a pool cue hung from two music stands standing either side of a pool table.
One YouTube viewer said these clips brought tears to his eyes, while another praised his film as "a masterstroke".
However, his family are a little shocked by the attention he has received.
"They know me, though," he said. "They know I do the slightly-odd thing," he said.
He will watch the final at Wembley with his father Colin, his brother Julian and his uncle Malcolm - who is flying in from Saudi Arabia.
A Lego spokesman said: "It's great to see how creatively Ashley has used the Lego mini-figures.
"He so clearly shows the only limit with Lego is imagination."
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