The 6th Amendment is about the Right to a fair Trial and it also addresses the subject of Witnesses. It is also called the Compulsory Process Clause or the Speedy Trial Clause.
Gideon was charged for breaking into a house. He ask the court to provide a lawyer because he was too poor to be represented. The court denied it so he defended himself and provide his own witnesses. But yet he was found guity and spent 5 years in jail.
Simmons was sentenced to death at the age of 17, he was mentally ill and argued that his 8th amendment rights are being violated (Cruel and unusual punishment)