A vehicle's fuel line carries fuel from the tank to the engine. As all fuel is highly flammable, any leak in the fittings that connect two sections of the line can be very dangerous, resulting in fires or even explosions. To avoid this danger, take all necessary steps to adequately seal these fittings so that they do not leak.
Select the proper fitting for that section of the line. For instance, only match an NPT fitting to another NPT fitting, and not to an NPTF fitting. Only use fittings made of appropriate materials, such as brass or stainless steel. Even if you can get a plastic fitting that seems to seal well, your fuel could cause the plastic to deteriorate, and extreme temperatures could cause it to melt.
Select a proper thread sealant. Some use fuel-resistant Teflon tape, but this is not advisable, as it tends to break into small pieces that get stuck in sensitive components in your engine. Instead, use a paste sealant.
Open the hose clamps with pliers and place them on both of the hoses that you intend to connect. You typically cannot get them on there after you have already connected the line. However, pull each clamp back away from the end of the hose to make it easier to get the fitting inside.
Apply paste sealant evenly to the threads of the connector fitting so that it seals the whole connection. Do not allow the sealant to get inside the fitting where the fuel flows. Push the fitting into the end of the hose and use your pliers to open the hose clamp again and close it around the section of tube where the connector fitting is. Repeat this on the same side of the connection if necessary.
Run the engine. Watch the connections carefully. If you see any drips, turn the engine off, disconnect the fuel line and repeat the process.