A closer look
Put the stress on the first syllable. In IPA, the raised mark /ˈ/ sits just before the stressed syllable it marks.
Shape the word as two parts: a strong first syllable, then a light, unstressed second one that follows it.
Mouth movements for the supported sounds
Verified target: /ˈsʌmθɪŋ/, stressed on the first syllable. Major British and American dictionaries transcribe it the same way, so one shape works for both varieties.
Start with the sound itself, not the spelling. The written th hides the trickiest part of the word.
Feel the stress mark /ˈ/ as your cue: it sits before sum, so that syllable carries the beat and thing stays light.