Det jag vill göra nästa gång är att breda smörgåsar hemma och ta med dem i en korg.

Here det is part of a structure used to highlight what the thing is:

Det jag vill göra nästa gång är att ...

A natural English translation is What I want to do next time is to ...

So det does not mean a specific it here. It helps build a sentence pattern that is similar to what I want to do is ... in English.

A more direct version would be:

Nästa gång vill jag breda smörgåsar hemma och ta med dem i en korg.

That version is less framed and more straightforward.

Because jag vill göra nästa gång is part of a subordinate/relative clause after det.

In Swedish:

• in a main clause, you often get inversion after something is placed first
• in a subordinate clause, the subject normally comes before the finite verb

So:

• Jag vill ... = correct here
• Vill jag ... would not fit this structure

That is why Swedish says:

Det jag vill göra nästa gång ...

not

Det vill jag göra nästa gång ...

Those mean different things.

Swedish often uses time expressions like this without a preposition and without the definite ending.

So you say:

• nästa gång = next time
• nästa vecka = next week
• nästa år = next year

Not:

• i nästa gång
• nästa gången in this meaning

So nästa gång is just the normal idiomatic way to say next time.

This is a very common pattern in Swedish:

Det jag vill göra ... är att + infinitive

It means something like:

What I want to do is to ...

After är, Swedish often uses an infinitive clause with att in this kind of sentence. Here, att introduces what the speaker wants to do.

So:

• är att breda smörgåsar ... = is to make/spread sandwiches ...

Leaving out att here would sound wrong.

Because breda smörgåsar and ta med dem i en korg are two infinitives joined by och, and they both belong to the same att.

So:

är att breda smörgåsar hemma och ta med dem i en korg

means:

is to make sandwiches at home and bring them in a basket

Swedish usually does not repeat att before the second infinitive when both verbs are part of the same structure.

Here breda smörgåsar means to prepare sandwiches by spreading butter and/or toppings on bread.

The verb is connected with spreading something onto bread. So it is a little more specific than simply make sandwiches.

Depending on style or dialect, Swedish speakers might also say things like:

• göra smörgåsar = make sandwiches
• bre smörgåsar = spread/make sandwiches
• göra mackor = make sandwiches, more informal

In this sentence, breda suggests the actual preparation of the sandwiches.

Often yes, but not always in exactly the same cultural sense.

A smörgås in Swedish can be:

• an open-faced sandwich
• a slice of bread with butter and toppings
• sometimes a more general sandwich, depending on context

So when English says sandwich, Swedish smörgås may sometimes feel more like an open sandwich than the typical two-slices-of-bread sandwich.

In everyday use, though, translating smörgåsar as sandwiches is often perfectly fine.

Hemma means at home. It describes location.

So:

• hemma = at home
• hem = home, toward home

Examples:

• Jag är hemma. = I am at home.
• Jag går hem. = I am going home.

In your sentence, hemma is used because the sandwiches are being prepared at home, not moved toward home.

Ta med is a fixed verb expression meaning take along or bring.

So:

• ta med dem = bring them / take them along

The normal word order is ta med + object:

• ta med dem
• ta med smörgåsarna

Ta dem med sounds unnatural in normal modern Swedish.

So it is best to learn ta med as a unit.

Because dem is the object form, like English them.

Here dem refers back to smörgåsar:

• smörgåsar = sandwiches
• ta med dem = take/bring them along

Compare:

• de = they
• dem = them

In spoken Swedish, many people pronounce both as dom, but in standard writing the distinction is still usually:

• de for subject
• dem for object

Because korg is an en-word in Swedish.

So:

• en korg = a basket
• korgen = the basket

If it were an ett-word, it would be ett, but korg is not one of those nouns.

I means in. So i en korg means in a basket.

That is the normal preposition if the sandwiches are placed inside the basket.

So the phrase means the speaker wants to prepare the sandwiches at home and then bring them along in a basket.

It is understandable and basically natural, but it has a slightly framed, written feel because of the structure:

Det jag vill göra nästa gång är att ...

A simpler and more direct version would be:

Nästa gång vill jag breda smörgåsar hemma och ta med dem i en korg.

You might also hear more everyday versions such as:

• Nästa gång vill jag göra smörgåsar hemma och ta med dem i en korg.
• Nästa gång vill jag göra mackor hemma och ta med dem i en korg.

So the original sentence is fine, but it is a bit more structured than the most casual way of saying it.