I lived in Germany as a child and speak fluent German. I’ve taught people German informally and semi-formally and also work to maintain my own proficiency. Here are some tips and materials I’ve produced.
For teaching friends and family I chose the Willkommen! series since it’s very good, exercises all four modalities – hearing, speaking, reading and writing – and is reasonably priced. The Willkommen! series also includes English explanations of grammar at the A1 but only German from the B1 level on. Immersion is essential for achieving fluency, but I’ve found exploiting an adult learner’s well-developed language skills accelerates learning in the earliest stages.