Rerolling in Fate/Grand Order is a common practice for newer players and players who don't like the direction of their current account. There're a few ways to manage a proper reroll that are easy to mess up with out prior knowledge. The things to look out for are what servants are already obtained, which servants are available, current events in the game, and the odds of acquiring a 4 or 5 star servant.
The player has some options for rerolling, and certain advantages come with each. Compared to a lot of other gacha games, Fate/Grand Order makes rerolling too tedious and time-consuming to be worth much.
Tutorial Roll
The tutorial roll is the most common roll to try again because it's available to every player, always occurs at the same time, and doesn't take too much time to repeat after starting a new account. The only downside to this choice is that there're no 5 star servants under the tutorial banner. The 4 star servants are a great starting few if the player manages to draw more than one. Drawing more than one of these 4 stars puts the player at a great advantage moving forward into the game.
Outside of personal preference, the best servants to aim for are Heracles, Emiya, and Carmilla as they're the most powerful of the 10 available servants. If one of those are obtained, and the other 4 star is a weaker one, don't start the reroll again because that is still a powerful draw from the tutorial roll.
Repeating this step until you draw a strong 4 star and another 4 star from the banner should only take a few tries since at least one is guaranteed. On top of that, the entire process takes around 30 minutes.
Rolling After The Tutorial
Overall, this practice is not recommended but has its perks. After completing the tutorial, the player receives saint quartz for summons and unlocks the summon screen after a few more missions past the tutorial roll. After unlocking the summon screen, the player can roll for a single try at obtaining a 5 star servant.
With a single roll, the odds of obtaining a 5 star servant are extremely low and the process is way too long to actually call for trying this. The length of the missions needed to be completed, coupled with the odds of obtaining a 5 star and the fact that 2 4stars should be pulled from the tutorial are too many factors to control. With all those factors in mind, this process would take around 16 hours on average. The player could obtain a lot more in 16 hours and probably a 5 star servant by normal means.
Another option is to pay real money and get more materials with that money after the player is confident in their tutorial roll. Spending money before confirming that's the account to keep would be a mistake since that can't be transferred to the next account so watch out. Fate/Grand Order is very friendly to free to play users so don't feel pressured to use real money.
Rerolling During Events
During certain times of the year, Fate/Grand Order runs events that grant players with special summon materials or increased chances at rolling certain servants. These can make for opportune times to invest in a reroll. It's important to still pay attention to the tutorial roll because those servants will provide a great foundation for the account, even if there's an event with great rates.
Using the extra materials from events and the increased rates for summoning strong servants, weigh the options with the amount of time it takes to get a substantial amount of summons on the account. To do this, check the details of each banner available and pay close attention to the materials provided on login and event missions.
Saving A Certain Reroll
Once the player has rolled all the servants they want and is satisfied with the progress, they can save their account by binding it. Binding the account ensures that the account can be loaded up again if the player decides to continue rerolling for optimal servants.
To bind an account, visit "My Room" and select issue transfer number. The transfer number will ensure that the account can be accessed again, even after the app is deleted. The most important thing to not is that the transfer number has only one use, so it must be refreshed and made again if used once.
Other Reroll Advice
The tutorial reroll is the best option in terms of convenience, time consumption, and it won't drive the player nuts. There are no missions that absolutely need a 5 star servant to be completed so having multiple 4 stars is more than practical.
Waiting for an event to start the game or reroll again is not recommended either because events are fairly rare in Fate/Grand Order. By waiting for an event, the player would simply miss out on the opportunities to roll the game offers by playing it, and not be able to get the great rewards since the account would be too new.
Fate/ Grand Order is not a game where rerolling is too important aside from getting the 4 stars from the tutorial and finding interesting characters. After doing so, enjoy the game and finding great servants will come naturally from there.