Normally the kitchen suppliers designer, surveyor, contracts manager measure up and make drawings.
Once layout is agreed with customer they can then be passed on to a contract fitter.
Price Work. Normally the contractor will price off of the drawings and submit a price to you. You add your % to fitting costs and present two prices to the customer one fitted and one not.
Over time you need to get a feeling for how long fitting takes so you can price for fitting, or hire a contracts manager/surveyor designer with experience with kitchens. This way you have a greater control over the job and better profits. Both ways are accepted by contractors. Payments to the contractor should be invoiced and paid in stages on inspected work and how many payments will depend on the size of the job, (time it will run for) and the over all price. These need to be agreed between you and the contractor before the job starts. It is important to hold a retainer until you and the customer have agreed that the finished job is to both your satisfaction. This way if it's not you have a hold on the contractor to correct or finish anything that is outstanding. Some contractors will carry themselves financially over the whole job and invoice you the whole amount at the end, but this is all negotiable, do keep that retainer though until everything has been signed off by the customer.
Day Rate. This is a payment for day rate should the contractor not work on his price, this rate the contractor stipulates before working for you. This does happen in the trade and is normally kept between a contractor and you. Day rate pay will normally happen if a customer wants extra work done while the fitting is underway or has a change of mind and then wants the fitter to change something. This way you can ask the contractor how long the work will take in front of the customer and can quickly work out a ball park price for the customer there and then. Sometimes trusting customers just say do the work and bill me later. In this case the extra work is done, your contractor tells you how long it took to do and you bill your customer and pay your contractor accordingly.
One word of advice if you get a good fitter look after him/her because they are hard to come by and will not hang around if they are badly treated. Don't get a reputation for mistreating your contractors if they are good.
I have been out of the game for a few years now and you may want to check if you would need to become CIS registered or not.
Hope this helps and of course you can change these payment structures to suit you and the contractor, nothing is ever set in stone... apart form expensive worktops.
