The question is fair: if prayer is central to Islam, Muslims need more than private guesswork. The answer is that Islam never presented the Quran as a book detached from a living Messenger; it gave the Quran, the Prophet (PBUH), and a worshipping community that learned from him.
The strongest form of the question
The objection can be put like this: the Quran commands prayer again and again, but it does not give a full step-by-step manual - how many units at each prayer, exactly what is recited, where the hands go, and so on. If Muslims had no teacher or role model, then prayer would become speculation. People could invent different forms and still claim to be following the Quran. So, the critic says, either the religion is incomplete, or Muslims are relying on something outside what God revealed.
That is the strongest version of the argument. It is not silly. A religion that commands daily worship must also provide a reliable way to learn that worship.
The Quran itself points to the teacher
The problem is that the objection assumes Islam is meant to work without the Prophet (PBUH) as teacher. The Quran says the opposite. It repeatedly commands believers to obey the Messenger, follow him, and take him as the practical example of faith.
There has certainly been for you in the Messenger of Allah an excellent pattern for anyone whose hope is in Allah and the Last Day and who remembers Allah often.
Quran 33:21
And We revealed to you the Reminder so that you may explain to the people what was sent down to them, and that they might reflect.
Quran 16:44
So the Quran is not embarrassed by the need for explanation. It builds that need into the religion. The Prophet Muhammad (PBUH) did not merely deliver verses and walk away; he taught, judged, purified, corrected, and demonstrated worship.
Prayer was learned by seeing and doing
The daily prayer was not reconstructed centuries later from scattered clues. The Prophet (PBUH) prayed in public, in congregation, day after day. His companions saw him stand, recite, bow, prostrate, sit, and give salams. They taught their families, students, and cities. This is why the basic structure of the prayer is known across the Muslim world despite differences in some details.
Pray as you have seen me praying.
Sahih al-Bukhari
This hadith is short, but it captures the method: prayer is a transmitted practice, not a private interpretation. The Quran gives the obligation and many elements of prayer - purification, direction of prayer, standing, recitation, bowing, prostration, remembrance, and fixed times. The Sunnah gives the lived form, order, regular units, and details.
What the argument does and does not prove
The argument does prove something against the idea that the Quran alone, with no Prophetic teaching and no inherited practice, is enough to define the full form of salah. On that point, the critic has noticed a real weakness in a Quran-only approach.
But it does not prove that Islam lacks a teacher or role model. It proves the opposite: Islam requires the Prophet (PBUH) as the authorized teacher of the Quran. Nor does it prove that every report attributed to him is accepted blindly. Muslim scholars developed careful methods for weighing chains, texts, public practice, and agreement among the early generations.
So the prayer of Muslims is not a guess. It is Quranic in command, Prophetic in form, and communal in transmission. The believer is not left alone with an abstract text; he is taught to worship Allah as the Messenger of Allah (PBUH) taught his companions.