How to offer instalments on a website you already have
You do not need a new site to take a deposit and a schedule. You need one more destination for the buyer who is not paying all of it today.
The schedule is a page, not a rebuild
Most sites that sell something expensive already do the hard part. They show a price, they explain what the buyer gets for it, and somewhere there is a button or a form. Adding instalments does not mean replacing any of that. It means adding one more destination, for the buyer who has decided yes but is not handing over the whole amount today, and that destination is a single page showing a set of dates and taking the first payment. Your words, your photographs and your booking emails stay where they are.
It helps to separate two things that get merged in the asking. One is the checkout: the page where somebody reads the dates, agrees to them, and has a card charged for the first time. The other is the schedule itself, the standing record of what is owed, on which day, against which card, which has to keep working months later on an ordinary Tuesday when nobody is watching. The first is what you add to your website. The second is the part you should never build yourself.
Three ways to add it without touching the rest
In rough order of how little you have to change:
A link. You set up the plan, you get a URL, and you put it behind whatever button you already have, or paste it into a reply to the person asking. This works on a site built fifteen years ago, on a site you have lost the password to, and on a one-page profile that is not really a site at all.
A button beside the price. The same destination, placed at the moment of the decision rather than two clicks away. If your site has a theme editor or a page builder, this is a block and some text.
An embedded piece, a calculator or the plan page inside a frame, so the buyer sees the amounts and dates without leaving. Useful when the price varies by what they choose, because the arithmetic is the thing they want and the thing they will otherwise do badly in their head.
Start with the link, even if you intend to end up embedded. A link can be live this afternoon, costs nothing to withdraw, and answers the only question that matters at the start: does anybody take it? Two weeks of real buyers tells you more about where the button belongs than any amount of planning the layout.
Where it goes, and what it has to say
Put it next to the price. Not on a separate payments page, not under a tab, not in a paragraph near the footer. Somebody looking at a number they are unsure about decides right there, and if the alternative to paying in full is not visible in the same glance, they close the tab and you never learn they were close. One line is usually enough: the total, then the first amount and the date it comes out.
Whatever the buyer can see before agreeing should include all of it. The full price. Each amount and the calendar date it will be taken, written out rather than described as monthly. What the first payment secures, and by when the rest has to have landed. What happens to the remaining dates if they cancel. And a plain sentence about the mechanics: each payment is taken from their own card on those dates, into your account, with nobody advancing anything to anybody.
Keep the hard part off your own website
The versions of this that go wrong are the handmade ones. A spreadsheet of card details is a liability you invented for yourself. A reminder you send by hand works for three buyers and fails at thirty, always in the week you are busiest. A form that says ask us about a payment plan is not an offer of one; it is an enquiry you now have to answer, negotiate and then chase.
What you want instead is dull and specific. The money moves on your own payment account, so the name on the statement is yours and refunds and receipts work as they already do. The dates are automated, so nothing depends on you remembering. A failed card is retried and somebody is told, rather than quietly becoming a shortfall you find later. And every booking carries one answer to one question: paid in full, yes or no. That column is the point of the exercise, and you can have it without rebuilding a single page of the site you already have.