Teachers get dressed by the kids in their classes, who in return give a suggested £1 donation. On 16 June, simply gather your dressing up items and get styled by the kids in your class - you can gather the items yourself or ask the pupils to bring some things in.
You could then go on to do fashion show with the other teachers - maybe invite the parents in as well, getting the kids to decide who has the best outfit. You could alternatively have a photo shoot, a 'best in class' award for the most creative outfit... there are many ways it can work depending on the amount of time you have to organise the day.
To get the most out of the day, we'd advise you to use the educational resources in the weeks or days before to explore why you are doing it. Then plan your fundraising event and use the Twinkl resources on the day itself.