Replace idler pulley

It's really pretty easy if you have the right tool. The sender requires a special socket. Buy a new sender from the auto parts store and rent or buy the socket from them. Disconnect the electrical connector from the old sensor. Then attach the socket to a ratchet and unscrew the old sending unit. Install the new one and reconnect the connector
Remove oxygen sensor with special socket Ricks Free Auto Repair Advice

if the leak is coming out the seal behind the tie rod bellows then replace the steering rack as special tools are required to change this and chances are the housing is worn anyways. if its the seal at the input shaft were the steering column connects to the rack then you can change this seal.

Ford Trans Cooler Line disconnect tool, then a wrench

There is no problem with reinstalling the Serpetine Belt. But what have you done about the pulley that is locked up?

You need to find the bad pulley or you will smoke a belt before you leave the driveway. Unplug the wire on the A/C clutch pulley if the compressor is locked up. You need to do this because the A/C will cycle when the dash is set to Defrost setting as well as on AIR.

The idler pulley is the spring loaded tensioner. It is common to lock up. If this pulley is bad, you can keep using the A/C and just replace the Idler pulley.

Check the Alternator, water pump, anything that turns to find the lockup. The timing is done inside the engine with a steel timing chain and is not a part of the Serpetine belt.

if you just broke the little plastic clip that snaps into the fuel line then those are replaceable

if you have the connector with the push buttons then most auto parts stores have replacement lines that you simply cut and splice together... no special tools are needed

The nuts are maybe rusted and now no wrench will fit, you may need to replace it all. There is no easy way, they usually need to be made and flared on the spot and bent to fit.;

Using either a serpentine belt tool or a long ratchet and a socket on the tensioner pulley bolt, turn the tensioner to relieve tension and pop the belt off. Remove the idler pulley bolt and replace the idler pulley. To finish it up simply reinstall the belt.

check for 12 volts at battery with engine off.

The check for 14 volts with the engine running. If not 14, alternator is not generating any amperage.

Well there's only part that has to do with the cooling system and serp belt - the water pump!! Maybe it's starting to seize or has too much play. Either way, when you change the water pump, replace the thermostat and flush the system very thoroughly, those 98-99's are notorious for rust build up. Make sure to pull the heater hoses off and flush the heater core too. After all said is done, purge the coolant and make sure there is heat from the blower. If not you're going to have to replace the heater core because that gets clogged sometimes when flushing.

An idler arm is a steering component found on vehicles without rack and pinion steering. Your referring to an idler pulley. To replace: remove the belt, remove the bolt from the center of the pulley, replace the pulley and reinstall the belt.