Brass cross fittings are used to create a cross-shaped junction in a pipe, allowing fluid flow in four directions. Ensure that the pipe ends are clean, smooth, and free from any burrs or debris before installing the fitting. Apply a suitable pipe thread sealant or thread tape on the male pipe threads to create a leak-free seal. Hand-tighten the cross fitting onto the pipes until snug, and then use a pipe wrench to make a final quarter to a half turn to ensure a secure connection. Avoid over-tightening the fitting, as it may damage the threads or cause leakage. Check for any visible leaks after installation and make necessary adjustments if required. Ensure that the cross fitting is properly supported and braced to prevent undue stress on the pipes.