Bathtub plumbing essentially consists of getting water into the tub via the faucets and then out through the drain, with an allowance for the overflow, if necessary.
The first connection in bathtub plumbing should be the drain, as the tub needs to be positioned over this and joined to it. You also need the control to open and close the drain, which is a rod that attaches to a lever in the tub. The standard size for a bathtub drain is 1 1/4 inches. You will need to fit a tube from the overflow of the bathtub into the drain.
You will need to run a pipe from both the hot and cold lines to the bathtub faucets. You can put these in place after dry fitting your tub in order to make bathtub plumbing easier. Using PVC fittings or compression fittings is the simplest way to accomplish this, and will give a firm connection to the faucets.
When you fit the faucets, be sure to put the seal between the faucets and the tub, and complete the bathtub plumbing by putting a thin bed of clear silicone caulk around the base of the faucets where they meet the tub.