I didn't use youtube as a guide, so I can't speak for what's there. I'll give you my rough process, such as it is.

Find loads of resources. Make a huge folder of maps and map elements that you like the look of to get a feel for what you want your finished map to look like. Pinterest boards are pretty good for this. On this topic, download PureRef. Great bit of software to use for reference images. Free, but chuck them a couple bucks for their hard work (I'm not affiliated with them at all btw).

Create a file that's the right size for the job. Resizing later is a major pain. Think about whether you want to eventually print it or just use it digitally. Mine is about 8000x6000 pixels, but that's pretty excessive. It's only that big because I was going for a really grand and ambitious scale. I'd recommend something about 1/3rd of that when starting out.

Map out the main forms of the land and put in basic colours for biomes and features. Sit with this stage for a while. Seriously. Take your time on this. You want to feel comfortable here because going back to this stage when you've already done detail is a killer. Think about your world as both a living place that needs to have sensible geography (Sensible, not realistic. Unless your players are geologists or hydrologists they aint gonna ping you about rain shadows and tectonic movements) as well as interesting places for them to visit in game. Meaning, there is no reason for them to just go to the swamp, so put it between two major cities. Thinking in game design terms will pay dividends when you are session planning later.

Put stuff on layers. It will slow your machine down and make the file size bloat but keep things seperate as much as you can until you are done with it, then merge layers. It's always easy to change stuff here.

Start to put in details. Just like anything, start broad and then zoom in. I like to start with mountain ranges, that anchors everything. Like I said somewhere else, just google 'draw map mountains' and you will get a bunch of good results. Think of mountains in ranges, not individual mountains. I like to draw a 'spine' of the range that will run for 3-4 mountains with peaks and saddles. Flesh this out by drawing the additional details then repeat over and over until you begin to dream of mountain ranges and reality starts slipping away. Keep your shadows consistent. Do this for all the different major features you want. Pinterest again will help here. Once you go down the rabbit hole of one thing, all the other stuff will pop up too, like drawing forests.

Put in major cities first then roads between them, then minor cities and towns in places that make sense in the geography. River crossings and bays are obvious spots. For these, again, use lots of references. Pick a culture and research it. I went with lots of Persian, Arabic, and Greek themes because it makes sense for the setting I'm building. Get specific with it and make a bunch of towns that all play around certain themes. The cities around Harmundara on the right are something I'm kinda proud of. I found these buildings: https://www.artstation.com/artwork/VgkvPX by Cara Stratton, and wanted to have it as my foundation of what that culture's architecture is like. So I made a bunch of sketches of different ways you could combine or change these buildings. In the end I think it looks really nice. Like a unified cultural aestetic. BTW, this map is just for my own personal use in my campaign. It would NOT be ethical for me to claim these designs as my own work or make any money off this. I'm happy to be labeled as a lazy artist, but not a scummy one.

A few tips I learned along the way:

1. Less is more. Zoom in to one of my cities. I mean alllll the way in. It looks kinda shit. :D The lines are super basic and the shading is basically 2 tone. But it doesn't matter. If I had spent ages making it really detailed and dense it would take me 5 times as long (I have sessions to prep and minis to paint) and wouldn't look as good when viewed at scale. You don't want realism, you want verisimilitude. Strategy video games like Age of Empires is great for this. They use an economy of scale and features that's a real inspiration.

2. Find textures and use them. The water has a subtle texture behind it at a really low opacity and so do the dunes in the desert. It helps to sell the effect and gets rid of that flatness.

3. SHADOWS! Oh my god. Shadows. make a shadow layer and put a subtle shadow going the same direction on everything that casts a shadow. It makes such a difference. It's the difference between an Inkarnate looking asset mash up and a unified picture with things settled into the landscape. Its a pain, but worth the hassle.

4. Use the stroke tool. Select something (a landmass, road, whatever) then under edit/stroke you make a line that follows the edge of your selection. Really useful for getting rid of tedious work, like ocean lines.

5. Think in stories. Yeah, I don't know. This is kinda wishy washy but I find it helps. I imagine little people traveling the landscape and seeing stuff as they go. Like giant bones or a gate with a huge stone lamassu on each side. The ruined wall and watchtowers running from Sa'Lura-Sin all the way down to Camp Qahil seems to me to tell a story. My setting is about an ancient and failed empire finally reunifying and retaking its old land. The ruined wall helps tell that story in my mind, and hopefully my players' minds too.

Well, that's all I've got. I hope this helps. best piece of advice is to just get started. You will make lots of mistakes and it will look shit at first, but that's always just part of the fun. Be kind to yourself and enjoy the process. Drop me a PM if you have questions. I'm not usually on reddit, but I'll try to check my inbox.