Jump rings are one of the most versatile findings used in jewelry-making. They come in all sizes and gauges.
Use two pairs of smooth chain-nose pliers (bent or flat-nose pliers work fine as a second plier).
Push ring open with right plier while holding across the ring with left plier. To close, hold in the same way and rock the ring back and forth until ring ends rub against each other or you hear a click. Moving the ring past closed then back hardens the ring and assures a tight closure.