1. Most people who use conditioners put it into the tank then add the new water.
2. You can remove your thermometer and get close to the temp, or go by touch, or just add cold. The last is similar to rainfall and mountain run off in the amazon, and this colder water can trigger fish to spawn.
3. Conditioner is not necessary if you are not changing large % of your tank at at time or if you let the water sit for 24hr.
4. For delicate fish, like Discus, I make up a 20g trash can worth of water the day before. I put a pump and heater in the trash can, then add chemicals to adjust the hardness, pH, and plant nutrients. The next day after removing 30% of my tank's water with a python, I connect a tube to the pump in the trash can and use it to pump the new water into my tank. This takes about 10min. The pump I am using is for sumps, so it is strong enough to pump all the water from trashcan up to the 75g tank.