This time, we made a letter withs and drew pictures using Japanese brush and ink.

On a letter tThey pasted an origami paper cut out in the pattern of a cherry blossom petal on their letters.

Besides the pattern, they added messages to their familyies back home.

GettTaking the lecturer's advice, "The words to write on the letter will come out spontaneously when you think of your family back home," each of them started to write "Spring has come.

or "I want to see you, maom."

However, all of their characters were written hard with devotion to their familyies, which seemed valuable to me.

I'm wondering whether they put them in the post box or hadve them on display in their rooms.

Anyway, I hope their hearts reach their familyies back home.

Last week, there was an event to introduceing Japanese culture to foreigners living in Japan was held as a part of a Japanese language class.

grammatically correct but this is smoother

This time, we made a letter with pictures using Japanese brushes and ink.

On athe letters, they pasted origami paper cut out in the patterns of a cherry blossom petals.

There are several foreigners, so there are several letters (unless there is only one letter that everybody is all pasting on, then only "letter" at the start is plural but all of them still pasted their own papers)

Besides the pattern, they added messages to their familyies back home.

"their family" implies they all are part of only one family
informally you might be understood though but this is better

GettTaking the lecturer's advice, "The words to write on the letter will come out spontaneously when you think of your family back home," each of them started to write "Spring has come.

"to take" + someone's + "advice"

or "I want to see you, maom/mum."

"Mam" sounds like "Ma'am" which is not how people call their mothers
"Mom" = american english "Mum" = british english

However, all of their characters, written hardtirelessly with devotion to their family, seemed valuable to me.

this is alright but I would say "all the characters" instead of "all of their characters" as this might be confusing with "character" meaning someone's personality/reputation. i would just say "the characters" since you already mentioned them writing kanji earlier.

The commas are mandatory if you are writing as it's difficult to understand these kinds of sentence structures without the commas. "Hard" is also not very easy to understand (I had to read this twice to figure out what it meant) and this adverb is better

I'm wondering whether they put them in the post box or hadput them on display in their rooms.

Present tense is better than present continuous since "I'm wondering" implies you are wondering about it now as you are writing this.
"to put + something + on display" set expression

This time, we made a letter with pictures using Japanese brushes and ink.

FThe foreigners who participated came from various countries including Vietnam, China, Bangladesh, and Finland.

GettUsing the lecturer's advice, "The words to write oin theyour letter will come out spontaneously when you think of your family back home," each of them started to write, "Spring has come.

"Getting" sounds awkward here

or "I want to see you, maom."

However, all of their characters written hard withwith effort and devotion to their family seemed valuable to me.

Anyway, I hope their hearts reach their familyies back home.

Last week, an events a part of a Japanese language class an event was held in order to introduce Japanese culture to foreigners living in Japan was held as a part of a Japanese language class.

I teachaught the class as a volunteer.

This event takes place a couple of times aeach year.

On a letters, they pasted origami paper cut out in the patterns of a cherry blossom petals.

Besides the pattern, they added messages to their family back home.

Some foreigners looked confused abouts to not being able to use a Japanese brush properly.

GettingWhen given the lecturer's advice, "The words to write on the letter will come out spontaneously when you think of your family back home," each of them started to write "Spring has come.

How's everything going there?"

or "I want to see you, maum."

or mom spelling depends on American/UK English

Some of the characters were either written in the wrong stroke order or were out of balance.

However, all of their characters written hard with devofilled with consideration to their family seemed valuablesentimental to me.

I'm wondering whether they put them if they ended up posting the post box or ham or just placed them on display in their rooms.

Last week, an event to introduce Japanese culture to foreigners living in Japan was held as a part of a Japanese language class.

Some foreigners lookedwere confused about not being able to use a Japanese brush properly.

GettFollowing the lecturer's advice, "The words to write on the letter will come out spontaneously when you think of your family back home," each of them started to write "Spring has come."

"How's everything going there?"

or "I want to see you, maom."

However, all of their characters were written hard with devotion to their family and seemed valuable to me.