Βασίζομαι πολύ στις σημειώσεις μου όταν κάνω επανάληψη.

Because βασίζομαι is the normal verb form for to rely on / depend on.

• βασίζω usually means to base something or in other contexts to torture, depending on usage.
• βασίζομαι σε... means I rely on... / I base myself on...

So in this sentence, Βασίζομαι is the correct form because the speaker means I rely on my notes.

στις is the contraction of σε + τις.

• σε = on, in, at, to
• τις = the for a feminine plural noun in the accusative

So:

• σε τις σημειώσεις → στις σημειώσεις

After βασίζομαι, Greek commonly uses σε to express what you rely on:

• βασίζομαι σε κάποιον = I rely on someone
• βασίζομαι σε κάτι = I rely on something

That is why we get στις σημειώσεις μου = on my notes.

σημειώσεις is the plural of σημείωση (note), so it means notes.

Here it is in the accusative plural, because the preposition σε normally takes the accusative in Modern Greek.

For this noun, the nominative plural and accusative plural happen to look the same:

• nominative plural: οι σημειώσεις
• accusative plural: τις σημειώσεις

After σε, you need the accusative form, so:

• σε τις σημειώσεις
• contracted: στις σημειώσεις

In Greek, possessive pronouns like μου, σου, του, της, μας, σας, τους usually come after the noun.

So:

• οι σημειώσεις μου = my notes
• το βιβλίο μου = my book
• η φίλη σου = your friend

This is one of the most basic word-order differences from English. English says my notes; Greek normally says the notes my.

πολύ here means a lot / very much, and it modifies the verb βασίζομαι.

So it tells you the degree of reliance:

• Βασίζομαι στις σημειώσεις μου = I rely on my notes.
• Βασίζομαι πολύ στις σημειώσεις μου = I rely a lot on my notes.

In Greek, πολύ often comes before the phrase it emphasizes, but word order can move around for nuance.

Greek usually does not need an explicit subject pronoun, because the verb ending already shows the subject.

• βασίζομαι = I rely
• the ending -ομαι here tells you it is 1st person singular: I

So εγώ is unnecessary unless you want emphasis or contrast:

• Βασίζομαι πολύ στις σημειώσεις μου. = neutral
• Εγώ βασίζομαι πολύ στις σημειώσεις μου. = I rely a lot on my notes (maybe unlike other people)

This is a time clause introduced by όταν, which means when.

• όταν = when
• κάνω = I do
• επανάληψη = revision / review / studying over material again

So όταν κάνω επανάληψη literally means when I do revision or when I review/study.

Greek often uses κάνω + noun where English might prefer a single verb or a different expression.

Because κάνω is a very common light verb in Greek, much like do in English in expressions such as do homework.

So Greek often uses:

• κάνω επανάληψη = revise / review
• κάνω μάθημα = have class / teach, depending on context
• κάνω δουλειά = work
• κάνω μια ερώτηση = ask a question

This is very natural Greek. Even if English uses a single verb, Greek may prefer κάνω + noun.

Yes, literally επανάληψη comes from the idea of repetition or doing something again. But in study contexts, it commonly means:

• revision
• review
• going over material again

So when students say κάνω επανάληψη, they usually mean they are reviewing what they have already learned.

Not with βασίζομαι in this meaning.

The normal pattern is:

• βασίζομαι σε κάποιον/κάτι

So you should say:

• βασίζομαι στις σημειώσεις μου

Using με would sound wrong here, because βασίζομαι specifically pairs with σε to mean rely on.

The sentence can be rearranged, because Greek word order is fairly flexible. The original version is natural:

• Βασίζομαι πολύ στις σημειώσεις μου όταν κάνω επανάληψη.

But you could also hear:

• Όταν κάνω επανάληψη, βασίζομαι πολύ στις σημειώσεις μου.
• Στις σημειώσεις μου βασίζομαι πολύ όταν κάνω επανάληψη.

These all mean basically the same thing, but the focus changes slightly.

• Putting όταν κάνω επανάληψη first highlights the time frame.
• Putting στις σημειώσεις μου first emphasizes my notes.

Yes. You can rely on a person, a thing, information, notes, experience, and so on.

Examples:

• Βασίζομαι στους φίλους μου. = I rely on my friends.
• Βασίζομαι στα δεδομένα. = I rely on the data.
• Βασίζομαι στην εμπειρία μου. = I rely on my experience.

So in your sentence, using it with σημειώσεις is completely natural.