For each number, I mark all cells where I know I can’t put it.
If one block, column or row has only one possible cell where a number can be, I put it there and then update all cells where I now know I can’t put it anymore.
This algorithm probably isn’t the fastest, but it’s easy to do manually and it has the advantage that you don’t need to backtrack.
For a long time, it used to be Square Root of Minus Garfield - I actually got my username from there.
Nowadays, it’s probably xkcd.
/opt/microsoft/powershell/7/pwsh