(Adobe Illustrator is REALLY hard to learn how to use at first. Illustrator is a vector-based program as opposed to a raster-based program like Photoshop, SAI, or FireAlpaca. Illustrator isn't something you can just learn overnight so don't feel bad if you think it is difficult!)
To put a PNG file into a drawing I usually just select the image and then paste it into my drawing. It's really as simple as copy and pasting, and will create a new layer that I pasted it on. PNG files are nice because white will appear transparent! So when I pasted the image of the diamonds into my background only the dark parts showed up, the rest was transparent!
So I looked into this program a bit more. From what I am seeing here in FireAlpaca, you can pretty much do the entire tutorial that I posted.
Honestly it seems a lot like SAI and probably has almost the same capabilities. A lot of the functions are the same, just named different things. Like in SAI, there's this thing called the 'Stabilizer' that will make your pen strokes smoother, and in FireAlpaca it's just called 'Correction'. I'm guessing this program is pretty much like how GIMP tried to copy Photoshop. From what I am seeing here in FireAlpaca, you can pretty much do the entire tutorial that I posted.
I honestly don't think that you really need that much improvement, your artwork is very good!
Blending modes are in FireAlpaca, SAI, and Photoshop. They are kind of like filters that people use on photography, but they can also be used in painting!
Usually what I do when I use blending modes is a create a new layer over another layer. I would then clip that layer to the layer below by selecting the 'Clipping' option. Now when I go to fill in this new layer that I have just created, I will not spill out any of my color on to another area (so I'm basically coloring in the lines). You can either use a brush to color in small sections with a new color, or use the paint bucket to fill the whole thing in.
It's probably going to be easier to explain this if I just show you some examples. Link to image!
This is a drawing I'm working on right now. The picture on the left is how it looks without any blending modes at all. The second picture is after I created a new layer and applied a Clipping Mask above his scarf and filled it in with a bright green color with the Paint Bucket Tool. The third picture is after I turned the blending mode to Multiply. The fourth picture is after I lowered the Opacity of the layer to 15% with the Multiply function still on. The fifth picture is back on Normal but with 15% opacity.
The other ones will have similar functions. Luminosity and Overlay typically make things lighter. I sometimes use this for highlights.
You might notice that the last two images (fourth and fifth on the right side) look pretty similar. The Multiply function actually allows some of that blue from his scarf to show through.
Blending modes are something that you really just have to try and play around with. I use them sometimes if I want to change the mood of a drawing (so I will fill the entire drawing with a layer of blue and then set it to Multiply to make it look like a night scene or something.
I hope that was a bit helpful. Sorry it took so long for me to respond too, I've been working my summer job and haven't had too much time. If you have any additional questions though please feel free to ask (I can also clarify more on this stuff too if it is confusing!).