Place your tongue tip near the roof of your mouth behind your top teeth. Push air through the narrow gap. No voicing.

Americans pronounce southern as SUH-dhern (/ˈsʌðərn/). Stress falls on the first syllable — keep everything else short and quick. You'll hear it in sentences like "The southern weather is rather worthy".
Record yourself saying "southern" and play it back. The mic stays on your device — nothing's uploaded.
2 syllables, 5 sounds. Tap a syllable to jump to its row, then explore each sound's mouth shape and how it's made.
Place your tongue tip between or behind your front teeth, turn your vocal cords on, and push air through the gap.
Relax your mouth and lift the tongue back and up. Keep the lips neutral.

Touch the tip or front edge of your tongue to the roof of your mouth behind your teeth. Air flows through your nose.

The textbook way isn't wrong — it's just not how anyone actually says it.
Stress falls on the first syllable, not the others. Stretch SUH — keep everything else short and quick.
Americans use a relaxed retroflex R — the tongue curls back rather than rolling. The R is one continuous sound with the vowel before it, not two separate sounds.