In most cases the window supplier will quote for supply and fix so they are responsible for the installation; your builder or yourself will need to treat them as any other tradesman in terms of scheduling. Note that when you place the window order delivery will be quoted in weeks, but this doesn’t allow for holidays and some European manufacturers close for a month in the summer.
If buying second hand or end of line windows you will need to rely on the expertise of your builder or a joiner. You will usually pay a deposit when the order is placed, often half the amount, then the rest upon satisfactory fitting of all of the units. Check they all open and shut properly and there is no damage to the glass or frames.
Remember that windows and doors are installed during the first fix which means that there is a lot of plastering, cutting and painting still to come. Covering the frames and handles in a protective material will save hours of laborious cleaning in the future. That said once second fix is finished, make sure to remove the protective coverings as the glue on some of the sheeting supplied by manufacturers can become very hard to remove if left for too long.