There were several methods used by the southern states to keep African Americans from voting. One method that was used was the poll tax. The poll tax required a person to pay a fee in order to vote. Since many African Americans were poor, they could not afford to pay the tax. As a result, they did not vote.

Another method that was used was the literacy test. The literacy test required a person to read and explain a part of the Constitution. Since many African Americans had limited education, many African Americans could not pass the test in order to vote.

The grandfather clause was used to exempt poor, uneducated white southerners from these requirements. If a person’s father or grandfather had voted before the Civil War, a person was exempted from these requirements. Most African American males were not voting before the Civil War.

Some states developed something which was called "the white primary." This was a way to get around the 15th Amendment. Laws were passed that declared the political parties to be private organizations. African Americans were excluded from joining these private political parties. This prevented those African Americans who could vote from having much of a say in the candidates who were selected for the political parties.

White southerners often intimidated African Africans into not voting. African Americans were threatened with losing their jobs or having harm done to their family if they voted. These forms of intimidation helped to keep African Americans from voting.

There were many methods used to keep African Americans from voting.

There were many ways that this happened (I am assuming you are talking about the time after Reconstruction when blacks had the official right to vote but were still not allowed to do so).  Some examples:

• Poll taxes -- the practice of charging people a tax to vote.  Since blacks were poorer on average than whites, this affected them more.
• Literacy tests -- laws were passed requiring blacks to prove they could read in order to be able to vote.  Many poor blacks were illiterate.
• Understanding tests -- these are like the literacy tests.  But here blacks had to show that they understood passages that they were required to read.  Both this and literacy tests could be made as hard as they wanted to by white supremacist election officials.
• White primaries -- in the South, only the Democratic Party could ever hope to win an election.  The Democratic Parties of many Southern states made their primary elections white-only.  They said this was legal because the Party was a private organization and could hold elections however it wanted.

These were the main ways of preventing blacks from voting.