God told us how he did it.  I take Him at His word.

When I was first reading Genesis 1, it seemed so strange that God went into what seemed to me the wordy, "It was evening and morning the first day."  "It was evening and morning the second day." etc.  Now I know why He said it that way.  He knew that people were going to say that He created over billions of years and He wanted to make Himself perfectly clear.  Yes, "Yom" (the Hebrew word for day) can mean an indeterminate period of time, but whenever it is used with a number, the word morning and/or the word evening, it always means a literal 24 hour day.  He worded Genesis 1 to make sure you had to 100% ignore His words to believe anything else.

There are also little details like the Earth being made before the Sun and stars (unlike in the Big Bang Theory) and plants being created before the sun (which works for 24 hours but not millions or billions of years.

Scientists that believe in a literal 6 day creation have done a much better job of predicting what evidence we will find than those who believe in the Big Bang and billions of years.  They've been saying  the Big Bang is settled science for many decades, but they have to change it every couple of years to make it sort of match the evidence.  The same is true of Darwinian Evolution (and its various spin-off forms of Evolution)

The more science I study, the more I think the Big Bang and evolution are a religion and anti-science story telling.  When I was in high school, a bunch of things didn't make sense, but I just assumed that there were things I didn't understand and the specialists had figured it all out.  Then I found out that almost all scientists are specialists.  They all assume someone else figured it all out (especially with evolution).  The  biochemists think the biologists figured out how life came to be.  The biologists think the paleontologist figured out how life came to be and the paleontologists think the biochemists figured it out.  The truth is that a cell is so complex and especially irreducibly complex that it is incomprehensible to believe life happened by chance.