I would love to do someones car off here .
Tiff - If you can supply hot water with the foam even better . Some say the snow foam can be ott but it definately has its place , if the vehicle is really dirty then it comes into its own. It will prevent some possibilties of marring n scratching the surface if you just went straight in with a wash mitt . Its elimination process , the better condition the surface the more you,ll warrant using methods of elimination ime. If your planning on machine polishing anyway some people aren,t to bothered with preventing scratching there is no right or wrong just preference. Make sure you rinse thoroughly when removing the foam residue.Me i like to use the snow its great fun if anything
You,ll also when claying have a chance in marring the surface especially if contaminated heavily.
What clay do you have ? , i have a selection from soft, meduim and hard depending on the job in hand. You can use plain water or a lubricating liquid to help the clay run as smooth as possible.
After your claying session be sure its smooth as silk , there are more stages of decontaminating . Iron deposite removal (tiny filings can embed just under the the uppermost surface , living near a railway can do this for instance. Also paint cleansers will still remove dirt within the surface even after claying its quite suprising.

Once the surface is in a condition you feel happy to continue working with you could either polish the suface to enhance the condition , you may need compounds to cut alot of defects , or just a light finishing polish to enhance the the gloss it may well be in very good condition anyway, it depends how far you want to go.
You could go straight into glazing the surface after claying to enhance the paint depth and gloss without any cutting of the paint and in the same time 'fill' the minor imperfections . These are great products and can be used after polishing to enhance that tage even more. After this you,ll be wanting to seal all your hard work with sealants or waxes the chioce is yours. Use both but only use wax over sealants .
If machine polishing its nice to have a paint guage , a good torch or halegon light, polish residue removers is handy during correction and keep cleaning the pads etc whilst you work along .
Its never ending if you want it to be.

hth
Mike