I remember building a wheel when I was about 15 years old, I was doing something silly riding the bike and bent the rim.
So off I went to the bike shop and got a rim. then started the task of swapping everything over
A hour later I could not work out why every spoke was a inch to long, so thinking I must have been given the wrong rim, I took it back to the shop.
I must say the man in the shop was very good, He noticed I had just put all the spokes straight and not crossing them over as you do.
He told me to copy the spokes in the back wheel and it will be ok.
So about 2 days later I got it right, well sort of, the front wheel never did quite run true after that