Learn Hardcore Russian: Сегодня я сделал новую запись в блокноте.

сделал is perfective. It presents the action as a single completed whole: I made (and finished making) a new entry today.
делал is imperfective. It presents the action as process, repeated action, or background: I was making / used to make a new entry today (sounds incomplete or like part of a longer story).

In this context, you want to say the entry is finished, so the perfective сделал is natural.

Why is сделал masculine if я doesn’t have gender?

In Russian, past-tense verbs agree in gender with the person or thing doing the action:

я сделал – I (male speaker) made
я сделала – I (female speaker) made

The pronoun я itself is not marked for gender, but the verb form reveals the speaker’s gender.
So if the speaker is male, he says я сделал; if the speaker is female, she says я сделала.

What case is новую запись and why?

Новую запись is in the accusative case, because it’s the direct object of the verb сделал (made what? → a new entry).

запись is feminine. Nominative singular: запись, accusative singular: also запись (same form for feminine nouns ending in -ь).
новую is the feminine accusative singular form of новый (new), agreeing with запись.

So:

nominative: новая запись (a new entry – as subject)
accusative: новую запись (made a new entry – as object)

Why is it в блокноте and not в блокнот?

This is a classic location vs. direction distinction:

в блокноте (prepositional case) = in the notebook, location, where something is.
- Used with в + prepositional when the meaning is inside / in / at.
в блокнот (accusative case) = into the notebook, direction or movement into something.
- Used with в + accusative when the meaning is into / to.

В блокноте tells us where the entry exists (in the notebook).
If you focused on the act of writing into the notebook, you could say:
Сегодня я записал это в блокнот. – Today I wrote this into the notebook.

Why is it в блокноте, not на блокноте?

В generally means in / inside; на generally means on / on top of or on (a surface / platform).

в блокноте – in the notebook, meaning inside the pages.
на блокноте would literally be on the notebook (e.g. written on the cover), and is not what you normally mean for an entry inside the notebook.

For entries written in a notebook, Russian uses в блокноте.

What case is блокноте, and how is it formed?

Блокноте is prepositional singular of блокнот (a masculine noun):

Nominative: блокнот
Prepositional (after в, на when indicating location): в блокноте

For most masculine nouns ending in a consonant, the prepositional singular ending is -е:
дом → в доме, город → в городе, блокнот → в блокноте.

Could I also say Сегодня я сделал новую запись в своём блокноте? What’s the difference?

Yes, that’s correct Russian.

в блокноте – in a/the notebook; whose notebook is clear from context.
в своём блокноте – in my (own) notebook, explicitly stating it’s your notebook.

Своём is the reflexive possessive (one’s own), usually preferred with я / мы instead of мой / наш in neutral style:
я сделал запись в своём блокноте is often more natural than в моём блокноте.

Why use сделал запись instead of a single verb like записал or написал?

All are possible, but they have different nuances:

сделать запись (в блокноте) – literally to make an entry (in a notebook). Very close to English make an entry / a note. Focus is on the fact of a new entry existing.
записать (что-то в блокнот) – to write something down, record information. Focus on recording.
написать (что-то в блокноте) – to write something in the notebook; more general write, not necessarily as a “formal entry”.

In many contexts these overlap, but сделать запись sounds like a discrete, completed entry in some record, notebook, or log.

Can I omit новую and just say сделал запись? Does it change the meaning?

Yes, you can say:

Сегодня я сделал запись в блокноте.

The difference:

сделал запись – made an entry (no comment on whether it’s new, additional, etc.; that’s usually implied anyway).
сделал новую запись – emphasizes that this is another / new entry, as opposed to editing or rewriting an old one.

So новую adds a small nuance of “additional, fresh”.

Is запись feminine or masculine, and how do you know?

Запись is feminine.

Clues:

Many nouns in -ь are feminine (though there are masculine ones like день, камень).
The adjective agrees in feminine: новую запись (not новый запись).

So you learn with practice and dictionary help, but the agreement новую запись tells you запись is treated as feminine.

How is the past tense сделал formed from сделать?

Infinitive: сделать (to do / to make, perfective).
Past tense is formed by taking the past stem and adding -л, then agreeing in gender/number:

masculine: сделал
feminine: сделала
neuter: сделало
plural (all genders): сделали

So я сделал = I (male) did / made.

Where else can сегодня go in the sentence? Is word order flexible?

Word order in Russian is fairly flexible. All of these are grammatically correct:

Сегодня я сделал новую запись в блокноте. (neutral; time at the beginning)
Я сегодня сделал новую запись в блокноте. (also neutral; “today” after the subject)
Я сделал сегодня новую запись в блокноте. (slight emphasis on made today, still natural)

More unusual orders are possible for emphasis, but these three are the most typical in everyday speech.

Could I drop я and just say Сегодня сделал новую запись в блокноте?

Yes, you can omit the subject pronoun in Russian when it’s clear from context:

Сегодня сделал новую запись в блокноте.

The verb form сделал already tells us the subject is singular, masculine.
Leaving out я sounds:

normal in informal speech when context is obvious,
a bit more diary-like or note-like, as if you were listing what you did.

Including я is more neutral and explicit.

What exactly does запись mean here? Is it like note, entry, record?

In this context, запись most closely matches:

entry (in a notebook, diary, log), or
note / written record.

Related words:

записка – a short note (often a message on a small piece of paper).
заметка – a note / remark / short article; also a personal note.
запись – a recording (audio/video) or a written entry in a list, log, notebook, etc.

Here, because of в блокноте, it clearly means a written entry in a notebook.

What is the difference between блокнот and тетрадь?

Both can be translated as notebook, but:

блокнот – usually a small notebook / notepad, often used for quick notes, to-do lists, phone numbers.
тетрадь – a school exercise book or larger notebook used for studying, homework, lectures.

In this sentence, блокнот suggests a personal, maybe smaller notebook rather than a school exercise book.

Where is the stress in the words сегодня, запись, блокноте?

сегодня – se-ВОдня
запись – ЗАпись
блокноте – блок-НО-те

Stress in Russian is important because it often changes vowel quality, so these patterns are worth memorizing.