Mount & Blade II: Bannerlord

To achieve an income of 10,000 denars a day, you must use one or a combination of the following:
Own profitable fiefs & castles, own profitable workshops, and/or own profitable caravans.

Honestly, in the early game (or if you are new to the game), it is hard to build profitable workshops and caravans, they require a bit more knowledge of the game as well as take a bit of time to develop.

I found it quicker to focus on my fiefs first and add in workshops and caravans last to bump up my income.

Once at clan level 2, become a vassal of any faction and fight for them, earn renown, influence, and relations within your faction to increase your chances of taking control of any castles or fiefs won in conquest. While still at war, garrison and protect your frontline fiefs as if you lose them and need to retake them, there is no guarantee you will win the vote to re-take ownership. I found once I hit my stride I was being offered castles and fiefs one after the other and after a successful campaign, I had 3 cities and 5 castles under my control.

Once at peace, we can start to build up those rookie numbers for your income. Hovering over your money icon should bring up a list of all the outgoings and incoming money. If you built up your garrisons during wartime (like I did), you will see these are massively draining your income. So the first thing I did was go to my clan tab, navigate to the parties sub-tab, scroll down to garrison parties and put a spending limit of about 200 denars per castle and 500 denars per city for recruitment. (If they are on or close to your current borders maybe leave them a little higher in case of war).
Next, we need to get the prosperity of the cities up. If possible, appoint a governor to each city so long as you have a clan member who is of the same heritage as this gives a +1 to loyalty per day. If your character's heritage and your governor's heritage don't match the city's locale, you will get -4 to loyalty per day, which is bad for prosperity. Early on, I'd say no governor is better than one that gives -1 while you are early stages. If you have the -3 loyalty for your character being of different heritage then clear the auto-generated build queue for that city and set the city to have perpetual festivities (one of the options on the bottom row) until you have 60+ loyalty, at which point build up anything that will boost prosperity for that town (this can be different in each city depending on the local stability, you might need to build hoses, or improve food production, etc.). Once you have built one or two things, return to festivities to keep the loyalty high.
Next, travel to your castles and do the same to the building queue, this time focusing on building the structures which reduce garrison costs first. I would only leave a clan member in charge of a castle if it was close to the borders of my kingdom, personally.
Next, looking at workshops, talk to anyone in a city (any city that is non-hostile, doesn't have to be your faction) whose name contains their trade, eg 'X' the Tanner, or 'X' the blacksmith. You can buy their workshop from them for usually upwards of 20k denars, but they are only worth buying if the outlying villages supply that city with the necessary raw materials (e.g. iron ore production in a village attached to a city with a blacksmith). I personally focus on the silversmith workshops or blacksmith workshops if I can as these seem to turn a profit quickly - although my experience overall is that great with workshops so far.
Lastly, Caravans. In the same way we bought a workshop, we can approach the same people to start a caravan, usually, for about 15k denars, don't pay for the 20k option as in my experience you never earn that cost back. Try to hire companions that have decent scout and trade skills for survivability out on the road. Caravans tend to start off with good earnings, but they can be attacked by bandits and in wartime, enemy factions, so I tend to rely on them less than workshops. In peacetime, try to clear up and bandit hideouts and start your caravans near factions you seem to never war with as much to provide your caravan with longer life (hopefully).

The final mentions are to war reparations. If you made peace with an enemy faction and they are going to give war reparations as part of the deal, I think this also counts towards your income. Also, in peacetime, try to reduce your active party to around 70-80 troops, dropping your elite (and more costly) troops if needed. Also, disband any clan parties you may have set up, these will both reduce party expenses.

Doing all of the above should get you at least near enough to 10k income, but if anyone has any further suggestions, please let me know and I can add your tips to the guide. If any of my points lack, a bit more depth, please let me know, I'm just fed up with typing at this moment! :)

You don't actually need 10.000 a day. Your income will add to the process to this achievement which is how I got it. Just set up some caravans or workshops, afrer a while you will get it.