One time, something happened that would leave Peter haswith a bitter memory of that event.

This is a better introduction to this little story.

Before his speech he was *guzzlingwolfing down* canapes, until one of them fell overnto his new white new shirt and stained it with sauce.

Guzzling is for drinks, wolfing down is for solid food. If you have to use the word guzzling for an assignment, you can say "Before his speech he was guzzling wine, until some of it splashed onto his new white shirt and stained it."

When listing adjectives in English, age comes before color.

He tried to *get across* the problem to one of his colleagues to switch theirand convince him to swap shirts.

But, although Pete was *loath* to try at first, his colleague convinced him it would be better to justo clean the shirt in the bathroom.

If you say Pete was loath to help, it sounds like Pete is helping someone with his own shirt, which doesn't make sense. I changed it so it means Pete doesn't want to try cleaning the shirt, but his colleague convinces him to try.

Peter using somescrubbed his shirt with soap.

If this is the caption under a picture, it's fine as is. If you want this to be part of the story, use the corrected version.

Peter didn't wanted people to *weigh up* his speech just forbased on the content and not be distracted by his appearance, so he tried itwanted his shirt clean.

Unfortunately, the stain didn't *wentcome out* and besideseven worse, the shirt wasended up *creased*.

Stains "come out" when they disappear. There is no reason for this, it's just how English phrases it. You could also say "The stain didn't go away", etc.