The United States Constitution has apportioned foreign policy powers to the executive (President) and the legislative (Congress) branches of the government. Article I of the Constitution enumerated congress' powers and one of them is that Congress has the power to "regulate Commerce with foreign Nations, and among the several States, and with the Indian Tribes". On the other hand, the Constitution left the power to make treaties and to appoint ambassadors to the President. Therefore, as sanctions are mainly about regulating commerce with foreign nations, it is fair to say that the power to impose sanctions is vested with Congress.

Yet, in the course of years following World War I, Congress gave broad powers to the President to regulate transactions with any foreign country. Such legislative authorization was first given in 1917 by the passage of the Trading with the Enemy Act (TWEA) of 1917 which granted broad powers in times of war. In 1933, Congress expanded TWEA by allowing the President to use powers granted by this act in peacetime. Later in 1941, following the breakout of the Second World War, Congress expanded the scope of TWEA by authorizing President the permanently seizure of certain properties. TWEA was used to impose sanctions against North Korea, Cuba, China, Vietnam, and Cambodia. 

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In the mid-70s, in order to push back on the President's use of national emergencies Congress passed two pieces of legislation to limit the use of extraordinary powers under a national emergency. The first was enacting National Emergencies Act (NEA) in 1976. This act terminated many existing national emergencies and put some restrictions on declaring new national emergencies. The second was enacting  International Emergency Economic Powers Act (IEEPA) in 1977. 

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To see more about IEEPA history you can take a look at a report prepared by Congressional Research Service titled "The International Emergency Economic Powers Act: Origins, Evolution, and Use".

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IEEPA is now the basis for all but one U.S. sanctions programs (the Cuba program is based on TWEA). Based on IEEPA, President may declare a national emergency in line with NEA procedural requirements, to deal with any unusual and extraordinary threat, which has its source in whole or substantial part outside the United States, to the national security, foreign policy, or economy of the United States. Based on IEEPA, President has the authority to: 

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(A) Investigate, regulate, or prohibit

(i) Any transactions in foreign exchange,

(ii) Transfers of credit or payments between, by, through, or to any banking institution, to the extent that such transfers or payments involve any interest of any foreign country or a national thereof,

(iii) The importing or exporting of currency or securities, by any person, or with respect to any property, subject to the jurisdiction of the United States;

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(B) Investigate, block during the pendency of an investigation, regulate, direct and compel, nullify, void, prevent or prohibit, any acquisition, holding, withholding, use, transfer, withdrawal, transportation, importation or exportation of, or dealing in, or exercising any right, power, or privilege with respect to, or transactions involving, any property in which any foreign country or a national thereof has any interest by any person, or with respect to any property, subject to the jurisdiction of the United States; and

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(C) When the United States is engaged in armed hostilities or has been attacked by a foreign country or foreign nationals, confiscate any property, subject to the jurisdiction of the United States, of any foreign person, foreign organization, or foreign country that he determines has planned, authorized, aided, or engaged in such hostilities or attacks against the United States; and all right, title and interest in any property so confiscated shall vest, when, as, and upon the terms directed by the President, in such agency or person as the President may designate from time to time, and upon such terms and conditions as the President may prescribe, such interest or property shall be held, used, administered, liquidated, sold, or otherwise dealt with in the interest of and for the benefit of the United States, and such designated agency or person may perform any and all acts incident to the accomplishment or furtherance of these purposes.

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So today, if a President would like to impose sanctions on a foreign government, group, or individual he or she needs to follow the requirements stated in National Emergency Act and within the powers granted under IEEPA. Here is an overall view of the steps needed to implement a sanction program: 

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