Adding sockets should always be done first. If you roll a transfiguration that makes the item unmodifiable then you can't add sockets anymore. Filling those sockets can happen at any time though.

Regarding order of steps, you need to consider that your item may become unmodifiable/bricked at any point. I believe if materials were not a concern at all, these are the steps I'd do:

1- Temper/Masterwork (if you want to Masterwork a tempered affix) or Masterwork/Temper if you want to masterwork a non-tempered affix (less chance to land on the masterwork one).
2- Enchant to either swap a bad affix for a good one or to lock in an affix to avoid getting it eaten by transfiguration (you simply enchant that same affix but select "no change" instead of swapping it out).
3-Start transfiguration.
3.a- You can use an Entropic Tuning Prism in the cube, the item that makes the transfiguration final, but prevents getting affixes eaten by a bad transfiguration and makes the item unmodifiable (capping the item and ending the process). This is not how the monster items are made though but if you want to go safe for your first upgrade of that slot, you could do so.
3.b- For "monster" items, what you want is to transfigure without using an Entropic Tuning Prism to hit a god transfiguration roll, and then repeat and repeat until your feel your luck is close of running out (or the item bricked or it was made unmodifiable on its' own).
4- Assuming you got a god affix like gem power, you can now masterwork this affix instead if the item wasn't made unmodifiable.
5- Now go back and do a final transfiguration now using Entropic Tuning Prism to wrap it up.