asita no ryokou no sitaku wo konya no uti ni site okimasu.

Each の links one noun to another.

• 明日の旅行 = tomorrow's trip / the trip tomorrow
• 旅行の支度 = preparations for the trip

So the whole chunk:

• 明日の旅行の支度 = preparations for tomorrow's trip

A good way to read it is from right to left in chunks:

• 支度 = preparations
• 旅行の支度 = trip preparations
• 明日の旅行の支度 = preparations for tomorrow's trip

Japanese often stacks nouns this way, and の is the tool that connects them.

In this sentence, 明日 is being used like a noun: tomorrow. To connect that noun to another noun, Japanese uses の.

So:

• 明日の旅行 = tomorrow's trip / the trip tomorrow

If you said 明日旅行, that would sound incomplete or unnatural in standard Japanese.

You can also think of it this way:

• 明日 by itself = tomorrow
• 旅行 = trip
• 明日の旅行 = the trip of tomorrow / tomorrow's trip

支度（したく） means preparation, getting ready, or arrangements.

In this sentence, it refers to the things you do to get ready for the trip, such as:

• packing
• checking tickets
• getting clothes ready
• preparing what you need

So 旅行の支度 means preparing for a trip or trip preparations.

A useful note: 支度 often has a practical, everyday feeling of getting things ready, while 準備（じゅんび） can sound a little broader or more formal.

They are similar, and in many situations both can be translated as preparation.

A rough difference:

• 支度: getting ready in a practical, often personal sense
• 準備: preparation in a broader sense; often more general

Examples:

• 出かける支度 = getting ready to go out
• 旅行の支度 = getting ready for a trip
• 会議の準備 = preparations for a meeting
• 試験の準備 = preparation for an exam

In your sentence, 支度 sounds very natural because it suggests actually getting everything ready for the trip.

今夜のうちに means something like:

• while it is still tonight
• before tonight is over
• by sometime tonight

The key part is 〜のうちに, which often means doing something within a certain time window before it changes.

So:

• 今夜のうちにしておきます = I'll do it tonight, before tonight ends

This has a nuance of not waiting until later.

〜のうちに means during, while still, or before the situation/time changes.

In this sentence:

• 今夜のうちに = before tonight is over

Other examples:

• 明るいうちに帰ります。
  I’ll go home while it’s still light.

• 熱いうちに食べてください。
  Please eat it while it’s still hot.

• 忘れないうちにメモします。
  I’ll write it down before I forget.

So here the speaker is saying they will finish the trip preparations within the time period of tonight.

The に marks the time limit or target point for the action.

• 今夜のうちに = by tonight / during tonight

Without に, the phrase would not sound complete here.

You can think of 〜のうちに as a fixed grammar pattern:

• noun / dictionary form / adjective + うちに

Examples:

• 今日のうちに = by today
• 若いうちに = while young
• 日本にいるうちに = while in Japan

So in your sentence, に is part of the natural grammar of 〜のうちに.

〜ておきます means doing something in advance, ahead of time, or for future convenience.

So:

• 支度をします = I do the preparations
• 支度をしておきます = I do the preparations in advance / beforehand

That is an important nuance in this sentence. The speaker is not just saying I’ll prepare. They are saying:

• I’ll get everything ready tonight so it will already be done for tomorrow.

That is exactly why 〜ておく is used here.

The pattern is:

• verb in て-form + おく

It means one of these, depending on context:

1. do something in advance
2. do something and leave it that way
3. do something for later convenience

In your sentence:

• する → して
• しておく
• polite form: しておきます

So:

• 支度をしておきます = I’ll make preparations in advance

Other examples:

• 切符を買っておきます。
  I’ll buy the tickets beforehand.

• 窓を開けておきます。
  I’ll leave the window open.

Because 支度をする is a very common expression meaning to prepare or to get ready.

Here:

• 支度 = preparation
• をする = to do

So:

• 支度をする = to do the preparations / to get ready

Then it becomes:

• 支度をしておきます = I’ll do the preparations in advance

This is one of those noun + をする patterns that are very common in Japanese, like:

• 勉強をする = study
• 準備をする = prepare
• 掃除をする = clean

The subject is omitted, which is very normal in Japanese.

From context, the natural subject is usually:

• I / we

So the sentence would naturally be understood as something like:

• I’ll get ready for tomorrow’s trip tonight.
• I’ll make the preparations for tomorrow’s trip tonight in advance.

Japanese often leaves out subjects when they are obvious from context.

Yes. しておきます is polite non-past form.

Breakdown:

• しておく = plain form
• しておきます = polite form

So the sentence is polite and neutral.

Plain version:

• 明日の旅行の支度を今夜のうちにしておく。

Past polite version:

• 明日の旅行の支度を今夜のうちにしておきました。

Yes, to some extent. Japanese word order is more flexible than English, as long as the particles stay attached to the right words.

The original:

• 明日の旅行の支度を今夜のうちにしておきます。

You could also say:

• 今夜のうちに明日の旅行の支度をしておきます。

That may even sound a little more natural in some contexts, because the time phrase comes earlier.

What matters most is keeping the chunks intact:

• 明日の旅行の支度を
• 今夜のうちに
• しておきます

A very natural English translation would be:

• I’ll get ready for tomorrow’s trip tonight.
• I’ll get everything ready for tomorrow’s trip tonight.
• I’ll make the preparations for tomorrow’s trip tonight.
• I’ll get ready tonight for tomorrow’s trip.

If you want to keep the nuance of 〜ておきます, a good translation is:

• I’ll get everything ready tonight in advance for tomorrow’s trip.

English often does not say in advance unless you want to emphasize it, but Japanese 〜ておく clearly adds that nuance.