Ég geri samloku með osti, gúrku og papriku fyrir hádegismat.

Yes:

• Ég = I
• geri = make / am making
• samloku = a sandwich (object form)
• með = with
• osti = cheese (after með)
• gúrku = cucumber (after með)
• og = and
• papriku = bell pepper / paprika (after með)
• fyrir = for
• hádegismat = lunch (literally midday meal, in the form required here)

So the structure is basically:

I make a sandwich with cheese, cucumber, and bell pepper for lunch.

Because geri is the 1st person singular present tense of the verb gera (to do, to make).

• að gera = to make / to do
• ég geri = I make / I am making

Icelandic verbs change form depending on the subject, so with ég you need geri, not the dictionary form gera.

Because samloka is the dictionary form (nominative singular), but here the word is the direct object of geri, so it appears in the accusative singular: samloku.

This noun is a common weak feminine noun, and those often look like this:

• nominative: samloka
• accusative: samloku

So:

• samloka = sandwich as the basic form
• geri samloku = make a sandwich

Because the preposition með usually takes the dative case when it means with.

So the nouns after með change form:

• ostur → osti
• gúrka → gúrku
• paprika → papriku

That is why Icelandic says:

• með osti
• með gúrku
• með papriku

A learner-friendly way to think of it is:

• með often forces the next noun into dative

Because Icelandic does not have an indefinite article like English a/an.

So samloku by itself can mean a sandwich, depending on context.

For the definite article (the), Icelandic usually adds it to the end of the noun instead of using a separate word.

For example:

• samloka = a sandwich / sandwich
• samlokan = the sandwich (nominative)
• samlokuna = the sandwich (accusative)

In your sentence, samloku is indefinite, so it means a sandwich.

In this sentence, paprika most naturally means the vegetable, that is, bell pepper / sweet pepper.

Because the sentence is about sandwich fillings, með osti, gúrku og papriku strongly suggests ingredients you put inside the sandwich.

The form papriku is just the case form required after með.

If someone wanted to make it especially clear they meant the spice, they would usually rely on context or use a more specific expression such as paprikuduft (paprika powder).

fyrir hádegismat means for lunch.

Here, fyrir expresses purpose or intended use: the sandwich is being made for lunch.

The noun is hádegismatur (lunch, literally midday meal), but after fyrir in this meaning it appears in the accusative singular:

• nominative: hádegismatur
• accusative: hádegismat

So:

• fyrir hádegismat = for lunch

Because Icelandic makes very frequent use of compound nouns.

Here:

• hádegi = midday / noon
• matur = food / meal

Together they form:

• hádegismatur = lunch (literally midday meal)

In the sentence, that compound noun is in the accusative form hádegismat.

This kind of one-word compound is extremely common in Icelandic.

It can mean either, depending on context.

The Icelandic simple present often covers both:

• I make
• I am making

So Ég geri samloku... could mean:

• I make a sandwich...
  or
• I’m making a sandwich...

If you want to make the ongoing action extra explicit, Icelandic often uses:

• Ég er að gera samloku... = I am making a sandwich...

But your original sentence is perfectly normal.

Yes. Ég geri samloku með osti, gúrku og papriku fyrir hádegismat is a normal, neutral word order.

A useful rule is that Icelandic main clauses are generally verb-second. In this sentence:

• Ég = first element
• geri = finite verb in second position

You can move things around for emphasis, but then the verb still usually stays second. For example, a more marked version could be:

• Samloku geri ég með osti, gúrku og papriku.

That sounds more emphatic or stylistically marked. The original sentence is the most straightforward version.

That comma is just punctuation separating items in a list:

• osti
• gúrku
• og papriku

This works much like English: cheese, cucumber, and bell pepper.

You may also see slight punctuation variation depending on style, but the grammar of the sentence does not change. The important part for a learner is that all three nouns belong together as things the sandwich is made with.