I know you can bake with it and there is word ta it explodes but how can I really use flour in these ways or more?

You tell the DM "I want to do X with the flour."  -- Replace X with whatever it is you want to do with the flour.

Once the action is agreed with DM , reduce quantity by one or more as appropriate on your sheet.

It’s not RAW, but there’s a common idea of throwing it on an invisible creature, or throwing it on the ground near an invisible creature to see its tracks. But, since it’s not RAW, those are basically specific examples of what cybermind said. Basically, whatever your DM will let you get away with. 

A common way that flour was used as an explosive was to dig a tunnel under a castle wall and suspend a table top from the ceiling by ropes. Then load the table up with loose flour and when the dust settled they would light a candle and position it under one of the ropes. When the rope burned through, the table would drop on that corner and the flour would *poof* into the air. Aerosolize flour is highly combustible and would explode, collapsing the castle wall. They used youths for the job because they were small, fast, and nimble and could navigate the tunnels more quickly than grown men. Needless to say, many of the youths who were used for this assignment didn’t make it. 

Yup, exploding flour.

Quote from Brian_Avery >>

Exploding flour?

Really?

PS. Throwing it in enemies eyes to temporarily blind them is an option.

 It's less that flour is inherently "explosive" than it is just a flammable powder. Anything in powder form that's decently flammable is dangerous... whether it's wood dust, flour, or even non-dairy creamer. So let's say a room is filled with flour... any slight motion kicks up a big cloud of the stuff that settles slowly. Every particle within that could could ignite, then spread the fire to the next nearest particle, and the next, and the next... the burst of heat and energy kicks up even more powder, which also ignites, and this all happens in an instant.

Look up exploding grain silo.

You should really see what non-dairy creamer can do. Wowza.

Sugar works even better. Even powdered Charcoal will work - just make sure you grind it very fine. Other things like chalk ground fine will work to show invisibles either by coating or tracks.  Most of my characters buy like 10 chalk sticks then grind up 8 and keep them in druggist folds of paper for ease of carry and disbursement. 

Keep in mind, per the RAW suggestion for explosives in the DMG optional rules, this would not be a particularly effective use of an action. Their suggested bomb will cover a 2x2 square and does 3d6 (~10 on average) fire damage on a save or suck DC 12 DEX save. Really, any kind of homemade explosives are subject to the "if your DM allows it" principle. Purely as a personal take, this is far enough outside the sphere of the core D&D premise that I'd save this for if an opportunity comes up naturally, rather than try and push to make it a go to option.

Flour is indeed extremely flammable.

Something else that hasn't been mentioned, but could be used for is for keeping track of where you have been.  Provided the environment isn't prone to draughts or breezes, you can leave a trail of flour to mark where you've been, or leave a small pile in the corner of a room so you know you've been there, if navigating a confusing dungeon.

In a campaign I ran, there was a cave full of mushrooms that gave off a poisonous spray when you walked through them covering the cavern floor.  One of the player characters (a dwarf) had two bags of flour on them.  He had resistance to poison, so what he did is tie the bags of flour to his back and made a small hole in each bag.  He ran through the cavern, smashing down a wide section of the mushrooms and leaving a trail that clearly marked the boundary of the path.  Because he got advantage on his saving throws, he never took poison damage.  The rest of the party waited about a minute for the spore cloud to disperse enough, then they followed the path as those spores had already been activated.  They never had to make saving throws and got through the cavern unscathed!

So, the thing is you need proper dispersion. 

For it to work, I'd say have a few sacks of flour, cast Dust Devil and have it pick up the flour, making a nice, dense cloud - then cast Fireball to set it off. 

Since I don't want such shenanigans at my table, I wouldn't allow it. But realistically, I say the explosion should do at least as much damage as the Fireball.

Quote from IamSposta >>

A common way that flour was used as an explosive was to dig a tunnel under a castle wall and suspend a table top from the ceiling by ropes. Then load the table up with loose flour and when the dust settled they would light a candle and position it under one of the ropes. When the rope burned through, the table would drop on that corner and the flour would *poof* into the air. Aerosolize flour is highly combustible and would explode, collapsing the castle wall. They used youths for the job because they were small, fast, and nimble and could navigate the tunnels more quickly than grown men. Needless to say, many of the youths who were used for this assignment didn’t make it. 

 When was that method ever actually used? How much flour would you need?

Some cooking uses for flour can be making gravy and use for cooking meats and etc, I believe you can make glue with flour just need the right materials. SOme pratical use for flour can polish steel and clean copper so you can have some fun rp moments with that, already see that people have mentioned the exploding property and the potential of revealing invsibile creatures, you can also use flour for traps and marking walls  in a dungeon I image a goblin tripping a trap getting covered in flour causing a ruckus. Ultimately be creative see what you and your dm can brainstorm makes it all the more fun.

paper mache is made out of flour...so maybe part of an impromptu disguise kit...just can't be used it the rain :o

also to just mark tunnels vs chaulk.

Quote from grobmaster >>

 When was that method ever actually used? How much flour would you need?

 it hasn't...flour has been mixed with real explosives though. https://www.military-history.org/back-to-the-drawing-board/back-to-the-drawing-board-aunt-jemima.htm   so throw in some smokepowder or gunpowder and you have something gnome-baker worthy.

and yeah, most dust is flammable - farmers experience grain dust fires in their combines all the time.

Flour looks awesome:

But I'd suggest that aerosolized oil catch fire might work better for an effective flame thrower effect, if it wouldn't mechanically break the game.

Quote from The_Ace_of_Rogues >>

Keep in mind, per the RAW suggestion for explosives in the DMG optional rules, this would not be a particularly effective use of an action. Their suggested bomb will cover a 2x2 square and does 3d6 (~10 on average) fire damage on a save or suck DC 12 DEX save. 

 Hate to be a guy who Isn't Fun, but this is the answer right here. Can it work? Yes. Should a bag of flour give you see invisibility, blindness/deafness and fireball all for the low, low price of 5cp? Of course not. The effect should match the cost.

Quote from grobmaster >>

Quote from IamSposta >>

A common way that flour was used as an explosive was to dig a tunnel under a castle wall and suspend a table top from the ceiling by ropes. Then load the table up with loose flour and when the dust settled they would light a candle and position it under one of the ropes. When the rope burned through, the table would drop on that corner and the flour would *poof* into the air. Aerosolize flour is highly combustible and would explode, collapsing the castle wall. They used youths for the job because they were small, fast, and nimble and could navigate the tunnels more quickly than grown men. Needless to say, many of the youths who were used for this assignment didn’t make it. 

 When was that method ever actually used? How much flour would you need?

 
From what I understand it was used in the Middle Ages before black powder made its way to Europe. As for how much, I cannot say. 🤷‍♂️