This video shows how to figure out the first number of the combination.
Part 1: https://www.youtube.com/watch?v=uurLUFlduU8
Part 2: https://www.youtube.com/watch?v=TdJLVDO9ipU
Part 3: https://www.youtube.com/watch?v=Bmofil8p9gg
Part 4: https://www.youtube.com/watch?v=kEINjfcI8Io
Most of the tricks in these videos were developed by others. Samy Kamkar has a particularly good web page with more details at https://samy.pl/master/master.html