The Torah gave us G-d's instructions of what to do (or not do). . .
For example: "you may slaughter of your cattle and of your sheep, which the L-rd has given you, as I have commanded you" D'varim / Deuteronomy 12:21.
As I have commanded you.
But the instructions of "HOW" G-d commanded this are not found in the written Torah.
Someone actually said to me "there is only one way to slaughter an animal."
This is patently false.
Animal slaughter is the killing of animals. Animals can be killed by suffocation, or by hitting them over the head, shooting them, wringing the neck (in the case of smaller animals), and so on. . .
And none of those methods are kosher.
A kosher animal is killed via שְׁחִיטָה / shechita (the laws of slaughtering). The animal is killed by a swift, smooth cut of a very sharp knife whose blade is free of any imperfection. If there is even a slight nick in the knife it cannot be used.
The trachea and esophagus must be severed according to the oral mitzvot. If this is not done properly the animal is not kosher and thus is unfit to be eaten.
There can be no hesitation as the animal is killed -- because that might cause pain or trauma to the animal. Chopping is not allowed. Burrowing the knife between the trachea and esophagus is not allowed. The cut must be made within a specified area -- or the animal is not kosher.
It is actually impossible to even read the written Torah without the oral Torah. . . There are no vowels in the Torah!! To even read Hebrew one must learn from the oral law.
Shabbat 31a tells the story of a non-Jew who came to the famous R' Shammai, saying to him "How many Torot (plural of Torah) have you?"
"Two,' he replied: 'the Written Torah and the Oral Torah.'
The non Jew said "I believe you with respect to the Written, but not with respect to the Oral Torah; make me a proselyte on condition that you teach me the Written Torah [only]."
R' Shammai scolded and rejected him in anger.
The gentile then went to R' Hillel who accepted the man as a student. On the first day, R' Hillel taught him the Hebrew Aleph-Bet (alphabet), beginning with Alef, beth, gimmel, dalet. . .
The next day the man returned and Hillel taught him the aleph-bet, but in reverse.
The man was confused and said 'But yesterday you taught me the opposite!"
R' Hillel explained that is the entire point -- no one can even learn the Hebrew aleph-bet without a teacher. Why rely on the teacher to correctly teach you how to read, and then not rely on the teacher with the respect of the oral law?
The Torah gave us G-d's instructions of what to do (or not do). . . but not how to do it.
Moses himself instituted courts and judges to try legal cases based on the mitzvot and laws given to us by G-d. D'varim / Deuteronomy 17:
"you shall come to the Levitic kohanim (priests) and to the judge who will be in those days, and you shall inquire, and they will tell you the words of judgment. And you shall do according to the word they tell you, from the place the Lord will choose, and you shall observe to do according to all they instruct you. According to the law they instruct you and according to the judgment they say to you, you shall do; you shall not divert from the word they tell you, either right or left."
If missionaries were right and "all you need" is the written bible, why did G-d command us IN THE WRITTEN TORAH to appoint judges and listen to them?
There would be no need!
Some convince themselves that there is no oral mitzvot by misreading the passage in Joshua which reads" "And afterward he read all the words of the instructions / הַתּוֹרָ֔ה / ha-torah the blessing and the curse, according to all that is written in the book of the Torah / בְּסֵ֥פֶר הַתֹּורָֽה / b'sefer ha-torah. There was not a word of all that Moses commanded, which Joshua did not read before all the congregation of Israel, with the women, and the little ones, and the strangers that walked among them." Y'hoshua / Joshua 8:34 -34.
Again -- every single mitzvah IS in the Torah. Not one is left out. We do not add to or subtract from the 613 mitzvot in the Torah.
Joshua read instructions are WHAT to do, not HOW to do them. This is what Joshua read to the people.