Roommate not paying rent

My roommate and I decided to get an apartment together, but she did not have enough credit, and the real estate agent was not comfortable having her sign their lease. So they had me sign the lease with the landlord, and then my roommate and I signed a private lease together that stated everything that the original lease stated, as well as the amount of money that she needs to pay me and late fees and everything. Rent was due in June 12th, and it is now August 3rd, and she still owes a little. Our next payment is due on August 12th, and she said that she will have it but I'm having my doubts. My question is, if she doesn't have it, since her and I are on our own private lease, can I give her a payment notice? I know that in my state (NH) I have to give a written notice for payment, but I cannot find anywhere that says how many days I must give her to pay. I was also wondering where I can find out how to get a written notice, is it something a lawyer has to write up or is it something each state has a copy of. I spoke with my landlord and she said that I would have to do this legally, and that she can't get too involved because since I am the one on the main lease, I have to pay the full amount, and then my roommate pays me and in this case I am the landlord to her, and she is the tenant. If anyone has any suggestions, or any information about written notices please let me know I am not sure where to take things from here. Thanks

Since she signed a lease with you, YOU are your roommate's LL. Go by the clerk of court's office and get a copy of a Pay or Quit notice. This notice is free (and may even be available on-line). This would be a 7 day notice in NH. Keep a copy of this notice for your records.