Я уже приготовила маринад и положила шампуры рядом с мангалом.

They are past tense feminine singular forms. In Russian, past tense verbs agree with the subject in gender and number.

So:

• я приготовила, я положила = a woman is speaking
• я приготовил, я положил = a man is speaking

The -ла ending tells you the speaker is female.

The dictionary forms are:

• приготовить = to prepare
• положить = to put, place

In the sentence, they appear as past tense feminine forms:

• приготовила
• положила

So you can think of them as I prepared and I placed/put, said by a woman.

Russian uses the perfective aspect here because both actions are presented as completed results:

• she has finished preparing the marinade
• she has finished putting the skewers next to the grill

If the focus were on the process or repeated action, Russian would normally use imperfective forms instead. Here the speaker is reporting completed steps, so perfective is the natural choice.

Уже means already. It shows that the action has happened by now.

In Я уже приготовила маринад..., it naturally comes after the subject я and before the verb phrase. That is a very common position.

Russian word order is flexible, so уже can sometimes move, but this placement is neutral and natural.

Маринад is the direct object of приготовила, so it is in the accusative case.

However, маринад is a masculine inanimate noun, and for many masculine inanimate nouns, the accusative singular looks exactly like the nominative singular.

So:

• nominative: маринад
• accusative: маринад

That is why the form does not change.

Шампуры is the plural of шампур and means skewers.

Here it is also a direct object, after положила, so it is in the accusative plural. But because шампуры is inanimate, the accusative plural is the same as the nominative plural.

So:

• nominative plural: шампуры
• accusative plural: шампуры

That is why it looks unchanged.

Because рядом с is a fixed pattern meaning next to / beside. The preposition с after рядом requires the instrumental case.

So:

• мангал = grill, brazier
• мангалом = instrumental singular

That is why Russian says рядом с мангалом = next to the grill.

Russian often leaves out repeated subjects when they are obvious from context.

So instead of saying:

Я уже приготовила маринад и я положила шампуры рядом с мангалом

Russian normally says:

Я уже приготовила маринад и положила шампуры рядом с мангалом

The second я is unnecessary because the subject is clearly still the same person.

Russian has no articles. There is no direct equivalent of English a/an or the.

So маринад can mean:

• a marinade
• the marinade

And шампуры can mean:

• skewers
• the skewers

The exact meaning depends on context. In this sentence, the context probably makes them specific, so in English you would often translate them with the.

Yes. Russian word order is much more flexible than English word order. The sentence as written is neutral and natural, but other orders are possible if you want to emphasize something.

For example:

• Маринад я уже приготовила... puts more emphasis on the marinade
• Шампуры я положила рядом с мангалом puts more emphasis on the skewers

The original version is a good standard choice for simply reporting what was done.