I posted this for someone on the construct subreddit, who was looking for a "general process" for making a game. It's a little long but hopefully helpful.
1) Create the core mechanics first with placeholder art in a test layout. Don't try to create any real levels yet. Ask, what is the player doing 90% of the time? If it's a platformer then that would be running/jumping around a test level. If it's an action game then that would be combat, etc. After this stage you'll basically have a toy to play with.
2) Then allow the player to accomplish the goals of a single test layout. It's important that you STILL don't create any "real" levels yet because you want to make sure your events are working correctly before worrying about the details of the actual levels. If you're a beginner, I would encourage you to design each level to have the same goal. In the first Mario game for example, 99% of levels is just to reach the end. That way you can create a system that can be re-used across all your levels.
3) Now create the challenges the player will face. If it's a platformer then make it so the player can fall down pits, get damaged by spikes, get hit by enemies, etc. Still do this in a test level. During this make it so the player can lose and re-play.
(Milestone: Core Loop) Those steps may take from a few days or months. Just depends on your skill level and how complicated your game is. After those things you'll basically have the "core loop" finished. The player can do the main things they can do, have challenges to face off against, and can win and lose. If you need a HUD or menus to get to this point, then go ahead and do it in a minimal fashion just to make everything work. It's important that all of this is done with placeholder art and definitely don't add any sound yet!
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4) Next you can add any supporting systems. Such as transitioning from level to level, collecting resources, power-ups, upgrades, crafting, etc. This will vary tremendously depending upon the kind of game you're making.
5) NOW you can start creating all your real levels and the flow between them. At this time you may also want to create the game's ending, whatever happens after they beat the final level. Create the title screen and any other menus like options, etc. Right now would also be a good time to add the ability to save the player's progress and be able to load it from the title screen.
(Milestone: Alpha) Remember, you did all of that with placeholder art and no sound. But you're entire game functions like it's suppose to. This milestone is called alpha.
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6) Now you can start creating / adding in all of your finalized art / animation / effects. This will take a really long time to tweak and make it "look right".
7) Then you can start creating / adding in all your finalized sound / music. This also takes a long time to tweak and get it to "sound just right".
8) After that you can pass through all of your content and fix bugs, balance your mechanics such as tweaking damage numbers, health points, speed values, pacing of each level. Then add polish to your art such as adding little effects like dust particles, atmospheric effects, making things shine and sparkle, etc.
9) At this point your game is pretty much finished. The vast majority of what you'll be doing is testing it, fixing bugs, adjusting the balance, getting feedback from other people, etc.
(Milestone: Gold) Once you feel really good about the entire game and you want to call it complete then you've reached the milestone called "going gold".
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10) I bet you thought you were done. But there's more! Now you must complete a few steps related to "publishing". You have to figure out how you're going to package and distribute your game. You need to create icons, splash screens, write descriptions, instructions on controls and installation. Are you just going to wrap up your game in a zip file? Maybe you want an installer to make it look a little more professional. Are you just going to email it to your friends? Host it on itch.io? Release on Steam?
(Milestone: Finished Game) Once you've worked through ALL of that........ Congratulations you've just completed your first game.