*playing on version 1.143
1. The easiest way to make money is to buy powder (found in quite a few cities) for less than 250 and sell at fortresses/castles for more than 750. This gets you starting money quickly (can also be used throughout the game).
2. Now you have some starting money. You can now either a. search for companions (guides are available for maximizing companion parties and builds) or b. attack small bands of looters/bandits/rebels in order to level up your trade (or obviously a, then b, etc.).
3. Once you have 5-7000 gold, search for expensive trade routes. For example, Azaq-Kale and the russian city next to it (Cherkassk) have lots of cheap iron you can sell for lots of money. They should have, between them, 15-20 iron. Before you buy, however, in the market menu, the top bar says "Asses prices". Click this, and after a little while, it will tell you the city with the highest price for your iron (if it's not there, just try again). You need to have at least 1 point in trade for this to work. Buy all the iron from one city and sell it at the other. Do not buy/sell more than enough to give you 20 iron in the city you are selling at. Talk to the mayor, and send a caravan, laden with 20 iron to the city with the highest price. Take as many guards as you feel necessary to protect it along the way. I usually minimize the guards I take with me, and defend it myself, but that's your choice. The caravan will more than pay for the small loss you took buying and selling at whatever city you bought the iron from.
4. Repeat these steps (buying goods from surrounding cities and selling them at one city in order to get 20 to send in a caravan) for any expensive goods you find. I've found good prices for iron at Azaq-Kale, tools at Krakov, Oil at Vilna, and Furs at Pskov/Moscow.
Using these steps, my dude now has reinforced Black Armor, a Good Miquelet Carabine, a Balanced Good Broadsword, rides a Heavy Courser, and has a fully armed and armored party of companions and mercenaries. About to unleash death on the enemies of Poland.