In BrE, it's "pay by (or in) instalments".
Note the BrE spelling too. Only one "l" in instalments. I believe there are two in the American spelling.
Thank you, emsr2d2. You have been very helpful.
Before the post was created, I tried several google searches holding the belief that to pay by installments should be the right one, but found some credible native-speaker-run websites adopted both ways, like : Pay by installment - Imaginative Traveller
OCAD - Students - Tuition & Fees - Paying by Installment
Do you think they are errors?