This should be a good procedure:
1 Settle the renting
Good advice would be not to cosign if you are not able to pay the whole rent. If you cosign the landlord can pick anyone of you and he has to pay him the whole rent. You then have to see how you are going to get the money from the others.
Generally it is better for you if one is the main renter and the others to pay him a portion of the rent. The main renter should be the one most likely to stay in the flat and should be able to pay the rent on his own for the time he can’t get out of the rental contract. This is not accepted by all landlords and he will usually try to get you to cosign.
2 Make a budget
You will need to work out how much you are going to spend on rent, additional cost (water, power, internet, tv), groceries, taxes ect. Then you have to figure out how much is to be paid by each one of you.
3 Set up an agreement
Now you guys should be in agreement on who pays what and how much. That's good. Write that down, sign it and everybody gets a copy. It's good to write things down when you are in agreement with each other.
Have a list of who owns what and who owes how much to the community.
Agree on what happens if one wants to move out or if someone new wants to move in. Also set up the rules you want to live by with each other.
Congrats! You have a contract with each other.
4 Set up a means to pay community expenses
I would set up a joint bank account. Get everybody a debit card and organize who is going to deal with the bills and the administration. Usually if one of you is more familiar with finances he will be a natural choice.
You now pay a regular cut of the budget into the joint bank account each month and with this money you will pay the expenses. The debit cards can be used to pay for groceries and the administrator will pay the bills directly from the bank account.
You will sometimes have money left over and sometimes there will not be enough and you have to readjust the budget.
You can also work with a expense sharing app like splitwise if you don't mind the shoveling around money all the time. This also can easily degenerate into one of you paying everything and the others owning him a lot, which tends to stress your relationship.
The steps above have to be taken regardless of how many you are or if you are in a romantic relationship, just friends or even relatives. Relationships can deteriorate and people will change so agree on things before you get into them and have a plan for when it doesn't work out.