Minha mãe usa farinha e manteiga para fazer o molho.

Because mãe is a feminine noun, so the possessive has to match it:

• minha = my, used with feminine singular nouns
• meu = my, used with masculine singular nouns

So:

• minha mãe = my mother
• meu pai = my father

Yes, A minha mãe is also possible in Brazilian Portuguese.

Both forms exist:

• Minha mãe usa...
• A minha mãe usa...

In Brazilian Portuguese, possessives with family members often appear with or without the article, depending on style, region, and level of formality.
Without the article can sound a bit cleaner or more neutral in simple example sentences.

So this sentence is completely natural as written.

Usa is the present tense of the verb usar.

Here it is:

• eu uso = I use
• você / ele / ela usa = you / he / she uses

So Minha mãe usa... literally means My mother uses...

In context, the Portuguese present tense can mean:

• a habitual action: My mother uses flour and butter to make sauce
• a general fact
• sometimes even a near-present situation, depending on context

Because minha mãe is third-person singular.

The verb usar changes depending on the subject:

• eu uso
• tu usas
• você/ele/ela usa
• nós usamos
• vocês/eles/elas usam

Since minha mãe = she, the correct form is usa.

Para fazer means to make or in order to make.

• para = for / to / in order to
• fazer = to make / to do

So:

• usa farinha e manteiga para fazer o molho = uses flour and butter to make the sauce

This structure is very common in Portuguese:

• para estudar = to study
• para trabalhar = to work
• para comer = to eat

Because after para, Portuguese usually uses the infinitive.

So:

• para fazer = to make
• para comer = to eat
• para estudar = to study

You would use faz only if it were the main conjugated verb of the clause:

• Minha mãe faz o molho. = My mother makes the sauce.

But in your sentence, the main verb is usa, and fazer is part of the purpose phrase: to make the sauce.

O is the masculine singular definite article, meaning the.

• o molho = the sauce

Portuguese often uses articles in places where English might or might not use them. In this sentence, o molho refers to a specific thing being made: the sauce.

Also, molho is masculine, so it takes o, not a.

Compare:

• o molho = the sauce
• a sopa = the soup

Because they are being used in a general ingredient sense.

• farinha = flour
• manteiga = butter

In this sentence, they function like materials or ingredients, so Portuguese often leaves out the article:

• usa farinha e manteiga = uses flour and butter

You could sometimes hear articles in other contexts, but here the version without articles is the most natural.

Yes, molho is a masculine noun, so it normally appears with masculine words:

• o molho
• um molho
• molho bom = good sauce

This is something learners often just have to memorize, because grammatical gender in Portuguese does not always match natural gender or English logic.

Mãe can be tricky because it has a nasal sound.

A rough pronunciation is something like mãyng, but not exactly like any normal English word.

A few points:

• ã is nasal
• the e at the end helps create a nasal diphthong
• the word is one syllable

If you know Portuguese nasal sounds, mãe rhymes approximately with:

• bem? No, not really
• pão? Also no, but it has that same nasal quality

A practical tip: say something like my but keep the vowel more open and let air resonate through the nose.

No, you do not need de here.

The normal pattern is simply:

• usar alguma coisa = to use something

So:

• usa farinha e manteiga = uses flour and butter

The sentence Minha mãe usa de farinha... would sound wrong here.

Yes, absolutely. That is a very natural alternative.

Compare:

• Minha mãe usa farinha e manteiga para fazer o molho.
  • Focuses a bit more on the ingredients she uses
• Minha mãe faz o molho com farinha e manteiga.
  • Focuses more directly on how she makes the sauce

Both are correct and natural. The second version may even sound a little more straightforward in everyday speech.

Usually it sounds habitual or general:

• My mother uses flour and butter to make sauce

That is how the simple present often works in Portuguese, just like in English. It often describes:

• habits
• routines
• general truths
• usual methods

If you wanted to make it clearly about right now, you would normally use another structure, such as:

• Minha mãe está usando farinha e manteiga... = My mother is using flour and butter...

But your original sentence most naturally suggests a usual way of making sauce.