Вечером я наношу крем на руки.

Yes. Вечером is the instrumental singular form of вечер (evening).

In Russian, some time expressions are commonly used in the instrumental to mean at/in a certain part of the day:

• утром = in the morning
• днём = in the daytime / during the day
• вечером = in the evening
• ночью = at night

So Вечером here functions like an adverb: in the evening.

Because Russian present tense is often used for habitual or regular actions.

So я наношу means not only I am applying, but also very naturally I apply / I usually apply.

With a time word like вечером, the sentence is understood as part of a routine:

• Вечером я наношу крем на руки. = I apply cream to my hands in the evening / In the evening, I apply cream to my hands.

If you wanted a single future action, you would normally use the perfective future instead.

Наношу is the 1st person singular present-tense form of the verb наносить.

So:

• наносить = to apply
• я наношу = I apply / I am applying

This verb is imperfective, which fits well for repeated, habitual, or ongoing actions.

Its perfective partner is нанести:

• Я наношу крем на руки. = I apply cream to my hands. / I usually apply...
• Я нанесу крем на руки. = I will apply cream to my hands. / I’ll put cream on my hands.

Yes, it could be omitted.

Russian often drops subject pronouns when the verb form already makes the subject clear. Since наношу already means I apply, the sentence could simply be:

• Вечером наношу крем на руки.

Including я is still perfectly normal. It can make the sentence a little clearer, slightly more explicit, or slightly more contrastive depending on context.

So both are natural:

• Вечером я наношу крем на руки.
• Вечером наношу крем на руки.

It is in the accusative, but for many inanimate masculine nouns, the accusative singular looks exactly like the nominative singular.

Here:

• nominative: крем
• accusative: крем

Since крем is the direct object of наношу, it is in the accusative case, but there is no visible change in form.

Because after на, Russian can use different cases depending on the meaning:

• на + accusative = movement toward / putting onto a surface
• на + prepositional = location on a surface

Here the idea is applying cream onto the hands, so Russian uses на + accusative:

• на руки = onto the hands

Compare:

• Я наношу крем на руки. = I apply cream to my hands.
• Крем на руках. = The cream is on my hands.

So на руки is correct because the cream is being put onto the hands.

Руки is plural because the sentence normally implies both hands.

Russian often uses the plural with paired body parts when both are meant:

• руки = hands
• ноги = legs / feet
• глаза = eyes

You could say на руку if you mean only one hand:

• Я наношу крем на руку. = I apply cream to my hand. / onto one hand.

But in ordinary everyday speech about hand cream, на руки is the most natural choice.

Yes. Russian word order is fairly flexible, and changing it usually changes emphasis, not the core meaning.

Your sentence begins with Вечером, which gives the time setting first:

• Вечером я наношу крем на руки.

Other possible orders include:

• Я вечером наношу крем на руки.
• Крем я наношу на руки вечером.

All of these can be grammatical, but the first one sounds very natural if you want to highlight when the action happens.

Yes. A very common alternative is:

• Вечером я мажу руки кремом.

This uses a different construction:

• мазать что чем = to smear/spread something with something

So:

• мажу руки кремом = I put cream on my hands / I cover my hands with cream

Compare the patterns:

• наносить что на что
  наношу крем на руки
• мазать что чем
  мажу руки кремом

Both are natural, but наношу крем sounds a bit more neutral or slightly more formal, while мажу руки кремом sounds very everyday and conversational.

Both can work, but they are not exactly the same in tone.

• наношу = apply
  This is a very standard, neutral verb, often used for cosmetics, medicine, creams, makeup, etc.
• мажу = smear / spread / put on
  This is more colloquial and everyday.

So:

• Я наношу крем на руки. sounds neat and standard.
• Я мажу руки кремом. sounds very natural in casual speech.

A learner should understand both.

A natural stress pattern is:

• Ве́чером я наношу́ кре́м на ру́ки.

A rough pronunciation guide:

• Ве́чером = VYE-che-ram
• я = ya
• наношу́ = na-na-SHOO
• кре́м = krem
• на ру́ки = na ROO-kee

The stress matters especially in:

• ве́чером
• наношу́
• ру́ки

Not exactly.

• вечером = in the evening / this evening / in the evening as a general time frame
• по вечерам = in the evenings / every evening / evenings generally

So if you want to make the habitual meaning more explicit, you could say:

• По вечерам я наношу крем на руки.

That sounds more clearly like a regular routine.
Your original sentence is still perfectly natural and can already be understood as habitual from context.