How do nucleotides form a double helix?
Nucleotides don't form a double helix. Single strands of DNA form a double helix, and single strands of DNA are made by nucleotides.
Nucleotides are monomers that make up the polymer called DNA. DNA is a single strand of nucleotides that are linked together. If you have a complementary strand (meaning a strand of DNA that can basepair to the first strand...G:C, A:T), then these two strands can form a double helix.