Feeding nipples into deeper section rims can be a pain. Old-timers will thread the nipple onto an extra spoke a turn or two and use that to guide it in to place, but there's still the risk of it unthreading and falling into the rim. It's happened to me more times than I can count.
Our nipple assembly tool is designed to carefully guide the spoke nipple to wherever you want it to go and hold it there while you begin threading it onto the spoke. It's spring loaded so it comes off the nipple easily, yet still holds it secure enough to start working with it.
Replacement springs are available and come in packs of 5.