Oczywiście wiem, jak zrobić domowy ketchup, ale potrzebuję jeszcze pomidorów.

Polish often leaves out subject pronouns when the verb already shows who is doing the action.

• wiem = I know
• potrzebuję = I need

The endings already tell you this is I, so ja is usually unnecessary. You would add ja only for emphasis, contrast, or clarity.

For example:

• Wiem = I know
• Ja wiem = I know / I’m the one who knows

Polish has two common verbs that both relate to English know, but they are used differently:

• wiedzieć → to know a fact, piece of information, or how to do something
• znać → to know a person, place, language, book, etc.; to be familiar with

Here the speaker says they know how to make something, so Polish uses wiedzieć:

• wiem, jak zrobić... = I know how to make...

You would use znam in sentences like:

• Znam Annę. = I know Anna.
• Znam ten film. = I know that film.
• Znam polski. = I know Polish.

Oczywiście means of course or obviously.

In this sentence it adds the idea that the speaker considers this information straightforward or unsurprising:

• Oczywiście wiem... = Of course I know...

It can move around depending on style and emphasis:

• Oczywiście wiem, jak zrobić domowy ketchup.
• Wiem oczywiście, jak zrobić domowy ketchup.

The first version is more natural and common.

Jak means how, and zrobić is the infinitive to make / to do.

So:

• wiem, jak zrobić domowy ketchup
  = I know how to make homemade ketchup

This is a very common Polish pattern:

• Wiem, jak to zrobić. = I know how to do it.
• Nie wiem, jak gotować ryż. = I don’t know how to cook rice.

So jak + infinitive is a normal way to say how to ...

This is about aspect, which is very important in Polish.

• robić = imperfective, focusing on the process or repeated action
• zrobić = perfective, focusing on completing something

Here the meaning is how to make a finished batch of ketchup, so zrobić is natural.

• wiem, jak zrobić domowy ketchup = I know how to make homemade ketchup

If you said jak robić, it would sound more like how to be making it / how to make it in general as a process, which is possible in some contexts but less natural here.

In Polish, adjectives usually come before the noun:

• domowy ketchup = homemade ketchup

So this is the normal word order.

Also, domowy agrees with ketchup in gender, number, and case. Since ketchup is masculine singular here, domowy is also masculine singular.

It is the direct object of zrobić, so grammatically it is in the accusative.

However, ketchup is a masculine inanimate noun, and for many masculine inanimate nouns, the accusative singular looks the same as the nominative singular.

So:

• nominative: ketchup
• accusative: ketchup

That is why the form does not change.

Because the verb potrzebować usually takes the genitive, not the accusative.

So:

• potrzebuję pomidorów = I need tomatoes
• not potrzebuję pomidory

This is something learners simply have to get used to: some Polish verbs require a particular case.

More examples:

• potrzebuję czasu = I need time
• potrzebuję pomocy = I need help
• potrzebuję samochodu = I need a car

Pomidorów is the genitive plural of pomidory.

Basic forms:

• singular nominative: pomidor = tomato
• plural nominative: pomidory = tomatoes
• plural genitive: pomidorów

Because potrzebować takes the genitive, you get:

• potrzebuję pomidorów

Here jeszcze means something like still, yet, or in addition.

So the idea is:

• I know how to make homemade ketchup, but I still need tomatoes
• or but I need some tomatoes first / as well

This word is very flexible in Polish. Depending on context, jeszcze can mean:

• still
• yet
• more
• another

In this sentence, the most natural idea is still / yet.

Yes. Polish word order is fairly flexible, and moving jeszcze changes the emphasis slightly.

For example:

• ale potrzebuję jeszcze pomidorów → neutral, very natural
• ale jeszcze potrzebuję pomidorów → emphasizes still
• ale pomidorów jeszcze potrzebuję → stronger focus on pomidorów or a more marked style

The version in your sentence is very natural and idiomatic.

Because jak zrobić domowy ketchup is a subordinate clause, and Polish normally separates such clauses with a comma.

So:

• wiem, jak zrobić domowy ketchup

This comma is standard in Polish punctuation.

In Polish, ale means but, and it is normally preceded by a comma when it joins two clauses.

So:

• Oczywiście wiem, jak zrobić domowy ketchup, ale potrzebuję jeszcze pomidorów.

That comma is required in standard Polish writing.

Not with exactly the same meaning.

• potrzebuję pomidorów = I need tomatoes
• muszę mieć pomidory = I must have tomatoes / I have to have tomatoes

Potrzebuję is the normal and most natural choice for expressing a practical need.
Muszę mieć sounds stronger and more forceful.

So in this sentence, potrzebuję jeszcze pomidorów is the best choice.

Yes, it sounds natural.

It has a very normal structure:

• Oczywiście = discourse word at the start
• wiem, jak zrobić... = standard way to say I know how to make...
• ale potrzebuję jeszcze... = natural contrast with but I still need...

A native speaker would understand it immediately, and it does not sound awkward.