Step 1; You don't. LOL WHO USES DECOYS WHEN YOU CAN USE FLASHBANG! NERD!
jk ;-)

Im gonna extend on that part tho^. Imagine a situation whereby you only have 150 dollars and you cant buy a flashbang cos you know, you dont have enough money. On top of that, its the last round.
In this situation, a decoy grenade would be useful because you can use up your money since its the last round anyways. You're probably asking, 'errm so? what do i do with it'

1) you can make use of it and use it as a 'decoy flashbang'.
if you watch tournaments and play competitive, you'll surely be familiarised with the 'pistol decoy'. Personally, i feel that a decoy grenade works better than a pistol decoy because the decoy grenade looks like a flashbang. It is useful on some occasions because the enemy might think its a flashbang and turn around upon seeing it. This gives you an upper advantage.

2) you can make use of it to fake your position.
If you have been to silver matches like i did, you'll be familiarised with the silver strat :throw decoy at a site and then rush the opposite site. Honestly i think that the chances of the enemy to fall for the trap is slim to none. BUT HEY! ITS WORTH A SHOT! theres still that 0.00000000000001 percent that it might be a success and who knows, you might even win the round if the opponent chooses to over rotate, furthermore , you can buy a decoy grenade equipped with armor in pistol round so... why not?

3) You can make use of a decoy grenade to get a kill
This goes out to all those people that uses the decoy grenades to headshot your team mates.
why would you even do that lol
anyways, you shouldnt do that to your own team mates because if you didnt know already, it lowers your team mate's armor. Imagine a situation whereby your team mate has no helmet because its the first round and you, trying to be a clown, threw the decoy grenade to your own team mate, lowering his armor. This makes 0 sense because your friend gotta rebuy his armor and this will affect the team's economy. SO DONT DO THAT. Instead, what you can do, however, is throw the decoy grenade to the enemy.
Imagine a situation whereby you have no more ammo for your ak and pistol. Youre left with no nades except the decoy grenade. Since youre a good aimer, and you learned the sacred airbending techniques of how to throw the decoy grenade to your own team mates, why not use it on the other team? [either that or reload your gun...BUT WHO CARES ]
The pros of this, you can send your video to SPARKLESSSS. and get famous

Yeah... thats about all that i can think of right now.... Not too shabby eh? at least it isnt useless
oh and if you havent realised it yet, the picture of this guide is actually a flashbang grenade, not a decoy. See what i did there? A FAKE FLASHBANG !!!!

PEACE... AND THANK YOU FOR SPENDING {WASTING} YOUR TIME ON READING THIS GUIDE .
I APPRECIATE IT ;-)