V and F are basically the same sound, except V is voiced. Alternate between them like VVVVVVVVVVVVVFFFFFFFFFVVVVVVVVVFFFFFF while touching your throat, you’ll feel the throat vibrate while saying V but not for F

voiced th is like this, that, mother

unvoiced th like thick, thimble, thirty

notice how the voiced th has a buzzing vocalization during the th sound, you can feel your teeth buzzing as you say the th in this

but when you pronounce thirty that buzzing is absent and the first buzzing starts with the i (the vowel is the first voiced part).

similar to th as in the English word thick, or a (usually apical) voiced alveolar non-sibilant fricative [ð̠],[2][3] similar to th as in the English word the

Should do the trick, no?