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I did it through trial and error. I put the factors of the numbers in so that I had a basic Sudoku. From there I picked out the rows or boxes with the least numbers missing in then used the grid to work it out. Another way I know works is fill in the factors then have a little number grid from 1 to 9 then work out as many 1's as possible then 2's then 3's and on. Then once you finish 9, the grid will not be complete so go back to 1 as there will be less alternative spaces for it to go with so it is easier. Repeat this until the whole grid is filled in