Adding a second hosting package, a domain or an add-on works the same way as your first order.
ORDERING
Use the Order menu, choose the product, and complete checkout. The new service attaches to your existing account, so your details are already there.
ADD-ONS TO AN EXISTING SERVICE
Services > My Services > click the service > Addons. Add-ons usually adopt the parent service's billing cycle, with the first charge prorated to align the dates.
BILLING CYCLES
New services get their own renewal dates. Over time you may accumulate several dates through the year. If that becomes a nuisance, open a ticket and ask about aligning them, which usually involves a short prorated adjustment.
BEFORE ORDERING MORE HOSTING
If you want to host a second website, check whether your existing plan already allows additional domains. Many do, and a second package may be unnecessary. Services > My Services shows your limits.
If you are hosting for clients rather than yourself, reseller hosting is usually a better fit than stacking sites into one shared account, because it keeps each client isolated.
Ask in a Sales ticket if you are unsure which route suits.