@Isabella Rose Your 45 days begins the day after the relinquished property is sold but prior to closing you must have established a Qualified Intermediary account. The QI will help advise and structure the 1031, create the taxpayer's 1031 Exchange account, prepare the necessary 1031 Exchange documents, securely hold the proceeds from the relinquished property sale, keep track of your 45 calendar day/180 calendar day deadlines (and also provide you with an identification form), and coordinate the 1031 with the closing agent. Keep in mind, a Qualified Intermediary can be anyone who is not a "disqualified agent". Like others stated above, your CPA or real estate attorney can't as your QI if they have provided legal or tax advice to you over the past two years.
Also, be aware that anyone can be a Qualified Intermediary. There is no government oversight in our industry and because of that there have been instances where QIs have not safeguarded their client's funds appropriately and/or they don't have the expertise and experience to adequately assist a client engaging in a 1031. Confirm the QI you chose is insured, bonded, holds client's funds in a Qualified Trust or Qualified Escrow account, has reputable reviews from others, have been in the business a number of years, and has the expertise and experience to assist you and your clients.