This is something I've done a LOT of in the past few years (for context, my company specializes in helping indies maximize the odds of raising publisher funding).

It ultimately falls down, in my opinion, to one deceptively simple think: Demonstrating intent.

That's easier said than done, but in essence, this is a filter through which you need to look at every action you take when putting together a deck, building a prototype, etc. More often than not, developers err on the side of ‘too much visual’, while in other cases, they fail to have the minimum viable amount of art. That's how a lot of prototypes end up looking like asset flips, or unfinished engineer class assignments.

The idea of demonstrating intent is that you have to be able to show just enough that the other party understands your Unique Selling Points (USPs) and sees why it will work. They don't necessarily a fully fleshed out inventory / crafting system if it isn't core to the visceral experience you're trying to put together. On the other hand, if this is a fighting game, chances are you need a minimal amount of polish on hit feedbacks for them to understand how it all makes sense.

There are three key questions you want to address, that they'll either ask out loud, or in their head:

→ Why this Game

It's understandably an important part of the pitch: why this game, how is it better? It can be some new original take on something, a cross of genres that truly has potential, a bunch of features that haven't really been tried together before, or purely better execution of a pre-existing game (though that can be a harder sell for the next question).

You must be clear and concise about your USPs so that they come out of there knowing the 2-4 reasons why this game can work.

→ Why this Team

As first-timers, this is the hardest question to answer. If this is a new team, composed of new people who haven't released games before, it will be VERY hard to truly convey why your team is better than another to carry this vision. What unique expertise, trait of character, or otherwise, do you possess that another team might not?

→ Why now

A lot of people forget to answer this one, and it is just as critical as the others. If someone was pitching me a platformer in 2021, I'd probably reply they're 5-6 years too late. Know the market, the trends, don't present a concept that's a ripoff of what's hot RIGHT NOW, look at what's warming up, or where the industry might be later down the road. Gap in the market? Niche that hasn't been catered to in a decade? Explain why now is the best time to be making this game and leave a lasting impression.

Something that helps is to help shape the ‘story’ of how you're achieving success one step at a time. Getting funding from family helps, but convincing strangers to give a few bucks, however little, helps even more. Kickstarter, Patreon, IndieGogo, etc. they're not really going to get your game funded in 2021, but they'll build a presence that becomes increasingly harder to ignore. If you can trace a path between day 0 and the moment where you ask for funding, and can show you've done the hard work AND achieved moderate success along the way, you'll stand a much bigger chance of landing said funding. Money attracts money, success attracts success, start small and build from there. When you've got 30k wishlists on Steam, 30k followers on twitter, etc. you'll know you can pitch; better yet, by that point, you'll probably have quite a few scouts headed your way asking about the game, sparing you a lot of ‘cold emailing’.

There's obviously more to this, and other replies to draw information from, but this should give you a fair assessment of where to start.