По этому образцу моя подруга быстро оформила плакат, а потом раскрасила его.

Here по этому образцу means according to this model/sample or using this as a pattern.

The preposition по often requires the dative case in Russian. One of its common meanings is according to, based on, or following something.

So:

• этот образец = this sample/model
• по этому образцу = according to this sample/model

Case forms:

• этот → этому
• образец → образцу

This is a very useful pattern to remember:

• по плану = according to the plan
• по правилам = according to the rules
• по примеру учителя = following the teacher’s example

Образец can mean sample, model, pattern, or example, depending on context.

In this sentence, it most likely means something like:

• a sample design
• a model to copy
• an example layout

So по этому образцу suggests that the friend made the poster by following an existing example.

Моя подруга is the subject of the sentence, so it stays in the nominative case.

• моя = my (feminine singular nominative)
• подруга = female friend (or girlfriend in some contexts, but usually female friend unless context says otherwise)

Because подруга is feminine, the possessive pronoun must also be feminine:

• мой друг = my male friend
• моя подруга = my female friend

These are past tense verbs, and in Russian the past tense agrees with the subject in gender and number.

The subject is моя подруга, which is feminine singular, so the verbs take the feminine past ending -ла.

Compare:

• он оформил = he designed/formatted
• она оформила = she designed/formatted
• они оформили = they designed/formatted

The same pattern applies to раскрасила:

• он раскрасил
• она раскрасила
• они раскрасили

Yes. Both оформила and раскрасила are perfective past tense forms, which present the actions as completed.

The sentence describes a sequence:

1. she finished designing/laying out the poster
2. then she colored it

The word потом = then/afterward also helps show the order.

So the sentence is not just about process in general; it tells us that these actions were carried out and completed.

They describe two different actions.

оформила comes from оформить and often means:

• to design
• to decorate
• to lay out
• to format
• to make something look finished/presentable

For a плакат, it can mean that she arranged the text/images, gave it a proper visual design, or prepared its overall appearance.

раскрасила comes from раскрасить and means:

• to color in
• to paint with colors

So first she created or arranged the poster’s design, and then she added color.

Плакат is the direct object of оформила, so it is in the accusative case.

However, for many inanimate masculine singular nouns, the accusative looks exactly like the nominative.

So:

• nominative: плакат
• accusative: плакат

That is why the form does not change.

Compare:

• Я вижу стол = I see the table
• Я оформляю плакат = I am designing the poster

But with an animate masculine noun, the accusative usually matches the genitive:

• Я вижу брата = I see my brother

Его means it here, referring back to плакат.

Since плакат is masculine singular and inanimate, Russian uses его as the object pronoun:

• раскрасила его = colored it

This avoids repeating плакат.

A learner should note that его can mean:

• him
• it
• his

In this sentence, it is clearly the object pronoun meaning it.

Both are possible in some contexts, but а потом is very natural here.

а often links two parts of a sentence while showing a transition or next step, not necessarily a strong contrast. In English, it can sound like:

• and then
• and after that
• then

So:

• оформила плакат, а потом раскрасила его
  = she designed the poster, and then colored it

Using а here helps separate the two stages of the action.

Быстро means quickly, and here it modifies оформила.

Russian word order is more flexible than English word order, so adverbs like быстро can often move around depending on emphasis.

This sentence uses a very natural order:

• моя подруга быстро оформила плакат

This emphasizes that the friend completed the design quickly.

Other word orders may also be possible, for example:

• Моя подруга оформила плакат быстро
• Быстро моя подруга оформила плакат

But they can sound different in emphasis or style. The version in the sentence is neutral and natural.

Yes, that is a good close translation.

A more natural English version might be:

• Using this sample, my friend quickly designed the poster and then colored it in.
• Following this model, my friend quickly made the poster and then colored it.

The exact English wording depends on how you want to render оформила, since that verb does not always have one perfect equivalent in English. For a poster, designed, laid out, decorated, or finished the design of can all work depending on context.