I don't use flux. Some fluxes are pretty corrosive and you don't want to be leaving them there long term to corrode your rails.

I use a 30W pencil or an 80W bolt. Both work fine. Boy does that 80W rig get a fast joint, but sometimes a few plastic ties start melting so after a bit of practice i prefer the 30W pencil that i also use for most wiring and electronics.

Far and away the most important thing is a clean tip. Tin it with a bit of solder (melt some solder on when it is real hot so it is shiny). Now let me give you the #1 all time secret for easy soldering: HAVE A DAMP SPONGE HANDY AT ALL TIMES. If you wipe the iron in the sponge before you solder you should have a nice satisfactory hiss of steam to tell you it is hot enough and a nice shiny clean tip for good heat transfer. Don't sizzle away too long ,you'll cool the tip.

If you do this you can use crappy cheap soldering irons, you don't need extra flux, you won't melt many ties.

For decades I tried to solder without a wet sponge. Once I learned this tip everything changed. I haven't hurled a solder iron at the wall since

That's all you need, but my other five top factors in easy soldering are:
2) Good solder. If you buy two or three different brandsa and mixes of rosin-cored solder you will be amazed at the difference in performance. I'm not in the USA so I can't recommend a brand. Try a few, find the best one, and stick to it.
3) Clean metal. This one has been covered. I use a file not a brush. I wipe the bottom of the rail, which also takes any burrs off so the rail joiner goes on easy. Oh yeah, don't try to solder rails without a joiner on. The joint will be a weak point and give you a jog in the track. Then I file in the web of the track. Everywhere the joiner contacts it. Then I use a screwdriver or blade to scrape the inside of the rail joiner to ensure some bare metal, no matter how shiny the joiner is. I suspect good quality rail joiners solder easier too but I can't prove that. I use Peco.
4) wait until the iron is hot. If it doesn't hiss on the sponge and solder soesn't melt as soon as you touch it to the iron, go do something else until it will.
5) good technique. Also been covered. Just remember the rail has to be hot. A bit of solder on the tip of the iron assists heat trasfer. Position the tip so as much of its surface area is touching rail as possible. This means pointing the bolt straight down so the end of it is on the top of the rail is NOT good technique. Hold the iron at a low angle and snuggle it in to the side of the rail.
Then wait a second until the ties start complaining , THEN apply the solder and watch it flow and get sucked into the rail joiner.
6) Wet tissues. Have two wet little balls of tissue. Put one an inch or so each side of where you are working, especially if working near points. This stops heat travelling too far down the rails if the job take s afew seconds longer than anticipated. If you are slick you shoudln't need them but...... Once the joint is made, blow on the solder until you see it lose its shine and harden. Then get a wet tissue on it to relieve the suffering of the ties.

Easy with a bit of practice. No flux. No fancy tools. Not too many melted ties.