First of all, transcription needs to happen inside the nucleus. An enzyme called RNA polymerase uses a strand of DNA as template for mRNA (messenger RNA) synthesis. The mRNA contains the codons. The next step of protein syntheis is called translation and occurs in the ribosome. Codons are groups of three letters that code for an amino acid. When the mRNA comes to the ribosome each codon pairs with an anticodon. Groups of three base pairs that are complementary to codons and are present in the tRNA(transfer RNA).
Each tRNA molecule has an amino acid attached to it and a specific anticodon. Depending on the order of codons proteins are build by binding amino acids.