1. Amending the Constitution
Article V of the Constitution provides two processes by which amendments can be proposed and approved

2. Article V
The Congress, whenever two thirds of both Houses shall deem it necessary, shall propose Amendments to this Constitution, or, on the Application of the Legislatures of two thirds of the several States, shall call a Convention for proposing Amendments, which, in either Case, shall be valid to all Intents and Purposes, as Part of this Constitution, when ratified by the Legislatures of three fourths of the several States, or by Conventions in three fourths thereof, as the one or the other Mode of Ratification may be proposed by the Congress; Provided that no Amendment which may be made prior to the Year One thousand eight hundred and eight shall in any Manner affect the first and fourth Clauses in the Ninth Section of the first Article; and that no State, without its Consent, shall be deprived of its equal Suffrage in the Senate.

3. Amendment Process Two ways = One has never been used. The first method
a bill has to pass both houses of the legislature, by a 2/3 of the vote each
Once the bill has passed both houses, it goes on to the states.
¾ approval
This is the route taken by all current amendments.
The second method
a Constitutional Convention called by 2/3 of the legislatures of the States, and for that Convention to propose one or more amendments.
These amendments are then sent to the states to be approved by 3/4 of the legislatures or conventions.
This route has never been taken.

4. Paths for an Amendment
Proposal by convention of states, ratification by state conventions
(never used)
Proposal by convention of states, ratification by state legislatures
Proposal by Congress, ratification by state conventions
(used once)
Proposal by Congress, ratification by state legislatures
(used all other times)

5. Ratification by state legislatures
Proposal by Congress
Ratification by state legislatures
Used 26 times
2/3
Used 1 time (21th Amendment)
3/4
Proposal by convention of states
Ratification by state conventions
Never used

6. Informal Amendment
The meaning of the Constitution, or the interpretation, can change over time.
Constitution changes through the judiciary.
a state forbids married couples from using contraception
A state forbids blacks and whites to marry
The Constitution did not changed but the way we looked at the Constitution changed

7. Reasonable time limit
ratification must be within "some reasonable time after the proposal."
Beginning with the 18th amendment, it has been customary for Congress to set a definite period for ratification.
18th, 20th, 21st, and 22nd amendments, the period set was 7 years,
33 proposals have obtained the necessary two-thirds vote in Congress.
Of the 33, 17 amendments have passed
Bill of Rights = created by 1st Congress ratified in 1791