אני כותבת על הלוח בטוש כחול, ואת אחר כך מסכמת במחברת.

That ־ת ending marks the feminine singular form in the present tense. So:

• אני כותבת = I as a female speaker
• את מסכמת = you to a female

If the speaker or the person addressed were male, you would usually get כותב and מסכם instead.

Here it means you feminine singular. Hebrew has two different words that are both written את in unpointed text:

• אֵת = the direct-object marker
• אַתְּ = you feminine singular

In this sentence, it must be the pronoun you, because it is the subject of מסכמת.

Because ו־ means and, and in Hebrew it attaches directly to the next word. So:

• ו + את = ואת
• ו + אני = ואני

So ואת simply means and you.

The definite article ה־ attaches to the noun, not to על. So:

• לוח = board
• הלוח = the board
• על הלוח = on the board

Some Hebrew prepositions do combine with ה־, especially ב־, ל־, and כ־. But על normally stays separate.

ב־ often means in, but it can also mark the instrument used to do something. Here it means with / using. This is very common with writing tools:

• בעט = with a pen
• בעיפרון = with a pencil
• בטוש = with a marker

So בטוש כחול means with a blue marker or using a blue marker.

In Hebrew, adjectives usually come after the noun they describe. So:

• טוש כחול = blue marker
• מחברת גדולה = big notebook

The adjective also has to agree with the noun in gender and number. Since טוש is masculine singular, the adjective is כחול.

אחר כך means afterward, later, or after that. It tells you when the second action happens. Its position is fairly flexible, so you could also say things like:

• ואת אחר כך מסכמת במחברת
• ואת מסכמת אחר כך במחברת

The version in your sentence is very natural.

In unpointed Hebrew text, במחברת can be ambiguous. It may mean either:

• in a notebook
• in the notebook

Context tells you which one is meant. If vowel signs were added, the two forms would look different, but in everyday writing they are usually spelled the same.