They are pretty simple, and pretty forgiving! Essentially, you’re blooming gelatin, adding very hot sugar syrup, and whipping the dickens out of it. There is a learning curve - overbeat, and your marshmallows can get tough, and may weep in storage, underneath and they may not set up quite as well - but at 20 min or so per batch (excluding setting time) and relatively low expense, it’s an affordable learning curve . Effort is greatly reduced with a stand mixer - a handheld will do the job just fine, but it’s hot work!
The first time I made marshmallow from scratch was with this recipe (they were delicious):
If you foray into adding spices/powders (like cocoa), read up; my first batch of cocoa marshmallows tasted delicious, but were pretty dense and chewier than expected. Next time, I followed a process that hydrated the cocoa, reduced liquid in the sugar syrup, etc and they came out much fluffier. I am still figuring out the best way to do matcha mallows; flavour was good on the ones I made, but the texture (for most of them) wasn’t quite right.
Hope this helps! I’m happy to help where I can, and there’s at least one excellent FB group dedicated to marshmallows too, if you end up wanting more info.