How to Attach Bait to a Fishing Rod？

First, upgrade your fishing rod and don't use a bamboo pole. To attach the bait, hold the stack of bait you want to select in your inventory and drag it onto the rod.
You can't do this in the toolbar; you must open the inventory.
You can choose the whole stack of bait or hold it for a while to count the number of baits you want to move.
Also, you need a fiberglass rod or better material to equip the bait. You can't equip bait with an old fishing rod.