Never done it, but here's some things to consider if you haven't already,
If you have a modern house and intend to use larger machines it is unlikely the floor will be able to withstand the weight / vibration from heavy tools. Even suspended floors on older house might not be up to it.
Vibrations can lossen old fatiqued plaster quite quickly.
If the workshop is against a party wall the neighbours might get annoyed and start the legal ball rolling. To effectively stop sound leaving any area you need to add mass to the dividing elements ie walls, ceilings and floors.
The cheapest method is to build inner drywalls inside the existing walls with a double layer of plasterboard facing. Contrary to their claims all lightweight materials (rockwool etc) do very little to stop sound leaving an area through a wall etc. Sound proofed rooms are also airtight - not good for simple dust control! They also have no windows.
Any sound proofing worth doing will be v. heavy. You would need to be sure the existing structure could safely take the weight.
Give us some more details if you still want to go ahead and I'm sure we could suggest a set-up based on the situation.
Thanks for the info Sam.
There won't be any heavy machinery, thats in a small workshop outside (run out of space to do any hand work), so machining will consist of routing, mortising etc. There's no party wall so soundproofing will be for the rest of the house.
This is just an idea as a interim solution until I build a larger workshop.