When you buy using Splitit, the retailer takes your first installment straight away, this may be later if your goods are being shipped at a later date. The rest of your monthly installments are automatically taken on the same date as your first one. This date cannot be changed. Monthly installments show as 'charged' or 'posted' on your account.
You don't need to do anything else.
In addition to your monthly installment payments, the full outstanding amount is authorized (held) on your card to guarantee future payments. Authorizations are usually renewed every 17-21 days until your plan is paid off. Authorizations get smaller with each installment paid. The previous authorization is released once a new one is obtained. Authorizations show as 'pending' or 'reserved' on your account. If your plan is kept on track, these authorizations are not charged.
You can check your monthly payments as well as whether a re-authorization is due (when and for how much), in the Splitit shopper portal.