Keep your tongue flat
Open your mouth
Keep your lip unrounded

“o”
Keep your lip round
Keep your tongue flinched
The root of the tongue should be slightly uplifted

“e”
Keep your lip open
Put your tongue backwards
Raise the root of the tongue

“i”
Keep your mouth open but not too big
Two of your lips should be flat
The tip of your tongue should contact with the lower teeth

“u”
Keep your lip open like a small circle
The tongue should be flinched

“ü”
The pronunciation of “ü” is basically the same as “i”, just keep the lips into a circle, slightly protrudes forward, the tip of the tongue should be up against the lower teeth

Try your best to remenber these tips by heart, and keep on practice, soon you will can pronounce simple finals of pinyin as well as native speaker.