1. Explain it simply
不但 introduces the first point; 而且 adds a second, stronger point.
This means 'not only... but also...', and the two points agree, not contrast.
Keep the same subject: subject + 不但 A，而且 B.
不但 pairs with 而且, not with 但是.
2. Show examples
这个饭馆不但便宜，而且好吃。
Zhège fànguǎn bùdàn piányi, érqiě hǎochī.
Sandhi:Zhège fànguǎn búdàn piányi, érqiě hǎochī.
This restaurant is not only cheap but also delicious.
不但 A，而且 B
他不但聪明，而且努力。
Tā bùdàn cōngming, érqiě nǔlì.
Sandhi:Tā búdàn cōngming, érqiě nǔlì.
He is not only smart but also hardworking.
two points agree
学中文不但有意思，而且有用。
Xué Zhōngwén bùdàn yǒuyìsi, érqiě yǒuyòng.
Sandhi:Xué Zhōngwén búdàn yǒuyìsi, érqiě yǒuyòng.
Studying Chinese is not only interesting but also useful.
second point is stronger
3. Share a tip
不但 pairs with 而且, not 但是. With one subject, put 不但 after the subject (他不但聪明), and make sure the two points point the same way.