Hi Kenn,
There is not an answer to each of your questions. Each item you mentioned can be by itself, or part of the process. What I mean is some guys only powder, some only chalk, some only spray, some only use washes and so on. I would think the majority of us use a combination, with no set pattern. If your interested in perhaps trying it out, May suggest the following;
1. Pick up a can of Testors "Dullcoat" spray from your local hobby shop. Now, a train shop that specializes in Only "O" gauge trains might not carry it. You" ll have to find a hobby shop or a shop that is geared towards HO scale, they (that shop) will have anything and everything you might want. Ask if someone there can help you with getting started in weathering, if not , look for another shop.
Start with a simple box car, a nice shiny one is a good start( not expensive), give it a couple of light coats of the Dullcoat spray. There, you've taken off the "shine" that made it look toy like. You're done! You've just weathered your first car. Don't laugh, sometimes simply "dulling" the shine is what some look for. Now, Step back, place it next to your other cars and see if it appeals to you.
If you like the direction that your headed, then go on the Internet to grab some pics of real railroad cars, pick one that is similar to what you like as far as how it looks from being out in the elements all these years. Same road name, car type, collr is very helpful, print the picture out, and head back to that HO hobby shop, tell them you'd like to try to make your car look like the one in the picture, in the simpliest cheapest way. A little suggestion, do not go for the heavily rusted, beaten up look right now, keep it simple. Just a slight faded look is a nice place to start.
He'll probably steer you into a powder assortment(perhaps 5 different colors/shades), should be in the $25-$35 range! Enough for hundreds of items. You"ll need a small assortment of cheap brushes(Walmart) and your ready to start weathering. Most beginners are very " heavy handed", nice thing is, there is no right or wrong. As Nike says, "Just do it" . Start with inexpensive cheap cars. Don't forget to weather the trucks!
Next simple thing are simple "washes". No trick here, just a thinned downed India ink.
For this, you can omit the Dullcoat step. The wash will help dull the surface. Cheap and easy to do.
Using washes , powders and Dullcoat should give you a good start. Check out YouTube, fellow modelers, Rich Basistas videos are all gret resources.
Keep us posted on how you did, maybe a pic or two.
