Мы решили оформить плакат наклейками и цветной бумагой.

After решить (to decide), Russian normally uses an infinitive to say what was decided:

• решили оформить = decided to decorate/design

This is very similar to English decided to do something.

Оформить is the perfective infinitive, while оформлять is imperfective.

Here, the speaker means a complete action: they decided to finish decorating the poster. That is why оформить is natural.

• решили оформить = decided to decorate it / get it done
• решили оформлять would sound more like decided to be decorating it, or decided to work on the process, which is less natural here

So the perfective verb fits the idea of a planned completed result.

Not exactly.

Оформить often means:

• to design
• to arrange
• to make something look the way you want
• to decorate in a purposeful or organized way

With плакат, оформить suggests giving the poster its visual design.

Украсить means to decorate more generally, often with emphasis on making something prettier.

So in this sentence, оформить плакат is closer to:

• design the poster
• decorate the poster using certain materials

Because плакат is the direct object of оформить, so it is in the accusative case.

For masculine inanimate nouns, the accusative singular looks the same as the nominative:

• nominative: плакат
• accusative: плакат

So:

• оформить плакат = to decorate/design a poster

If it were an animate masculine noun, the accusative would usually look different.

They are in the instrumental case because they show what was used to decorate the poster.

So the meaning is roughly:

• decorate the poster with stickers and colored paper

Russian often uses the instrumental without a preposition to express the material, means, or tools involved in an action.

Forms here:

• наклейка → наклейками (instrumental plural)
• цветная бумага → цветной бумагой (instrumental singular)

Because the speaker is thinking of:

• stickers as separate countable items → plural
• colored paper as a material/substance → singular

This matches English pretty well:

• with stickers
• with colored paper

Russian often uses singular for materials like бумага, краска, ткань, etc., when talking about them in a general material sense.

Because the adjective has to agree with the noun in case, number, and gender.

The base form is:

• цветная бумага = colored paper

But here the phrase is in the instrumental singular, so both words change:

• цветная → цветной
• бумага → бумагой

So:

• цветной бумагой = with colored paper

Yes. Russian often drops subject pronouns when the meaning is clear from the verb form.

So both are possible:

• Мы решили оформить плакат наклейками и цветной бумагой.
• Решили оформить плакат наклейками и цветной бумагой.

The version with мы is a little more explicit and can add emphasis or clarity.

Yes. Russian word order is fairly flexible, though different orders can shift emphasis.

The original sentence is neutral and natural:

• Мы решили оформить плакат наклейками и цветной бумагой.

Other possible orders:

• Мы решили наклейками и цветной бумагой оформить плакат.
• Плакат мы решили оформить наклейками и цветной бумагой.

These are still understandable, but the original order is the most straightforward for a learner.

The dictionary form is наклейка.

Here are the forms:

• singular: наклейка = sticker
• instrumental plural: наклейками = with stickers

So the sentence uses the plural instrumental form because the poster is being decorated with stickers.

In this sentence, the version without с is the natural one:

• оформить плакат наклейками и цветной бумагой

That means decorate the poster with stickers and colored paper.

Using с would usually change the structure or meaning slightly, and it would not be the most standard phrasing here. The bare instrumental is the usual way to show the material or means used for decoration.