Don't forget to get all your permits, and approvals first, and file all your applications with your utility, electric and if gas powered, with gas company, this is necessary to find out if the existing supply is large eniough, then don't forget, your city, or town, may need zoning approval, both for location of gen, and noise emmisions, then there is the building department, they can get wired if the pad does not meet their specs, then one of the most important things is, make sure your general liability covers generator installations, usually the more expensive contractor liability policies, with E&O do cover this, if your paying less then $4K for your insurance, you most likely don't have it, then make sure you have a way to unload the delivery of the gen, this can be a a real treat, if your not prepared.
Make sure your charging eniough, to cover all the extra time, and expenses, the guys that do these installs, on a regular basis know all the pit falls, and get a good dollar.
Takes us 4-6 hours with 2 guys. Another person takes 3-4 hours to help unload and do the gas piping. Many times we have to remove the 30' flex and pipe it in EMT. Also figure an hour for sales and site evaluation. Need to pick up a car battery and maybe gravel. Also may need to have someone around 1/2 the day to accept delivery. Allways let the customer wait for the inspectors. Unless you have other jobs close by you can work for just a few hours it will take up your day so charge for 1 day.