OK - The process from start to finish:
1) Planning - you will need decent working drawings from an architect, as a '***-packet' design will not wash with a conversion - these are harder to do than a new build.
2) Clearance - depending on the soundproofing spec specified by your architect, you will probably need new floors on the party floor area. Unless you want to make a real hash job I would reccomend hacking off all the plaster at this point too, as you are inevitably going to disturb the old stuff anyway and then have to do patching in, as well as mixing new plaster on ceilings with old on walls etc. Now is a good time to have a good look at the state of the timbers and carry out any remedial works. Organisation at this stage is of utmost importance, as if the clearance is essentially a forgotten element and is only organised on a day to day basis the costs will spiral massively.
3) Structural work - any walls that need doors cutting through etc, plus the carcassing for studwork should be put onto noggins between joists at this point (no more putting studs straight onto the floor I'm afraid, the flanking will make it fail its sound test).
4) First fix wiring and plumbing - you will need to apply for and have installed another gas, electricity and water supply to serve the additional dwelling you are creating. This is also the point when you should install any new drainage specified in the working drawings.
5) Soundproofing - this will probably consist of a gridwork ceiling, gyproc plank, insulation, and double fireline board with a skim below. If your ceilings are pretty character ones that you have any qualms over destroying you can forget converting the place into flats. I'll share a nugget of information with you that will save you a fortune too - whatever insulation your architect specifies is probably going to be manufactured by British Gypsum; go to your local builders merchant (not one of the nationals), and ask them for the generic alternative which has the same density. If you are employing a builder to oversee the subcontracting, you should specify in your contract that he is liable for remedial works should you fail your pre-completion test under Part E of the building regulations; and make sure they have done one to modern standards before - conversions are not to be taken on lightly.
6) Laying new floors - self explanatory really - but makes a heck of a difference to the feel of the place.
7) Plasterboarding and skimming - make sure you get a quote for this on a £/sq.m price; get at least three quotes or alternatively let your contractor do it for you and add a 25% markup. When they do the ceilings make sure the boards overlap so there are no gaps that sound can get through.
8) Second fix carpentry - Self explanatory really; doors, skirtings, architraves etc.
9) Decoration - Berger is the best paint going for fresh plaster, it's dirt cheap and look like Dulux. Roll it on very lightly and you only need one coat.
10) Second fix electrical and plumbing
11) Kitchen fitting
12) The sound test - must be done without carpets in place. Once you pass this, your new flats will be signed off as complete by building control and will be legally saleable.
"The power of accurate observation is commonly called cynicism by those who have not got it"