Find the actual boundary
Look for a written workplace dress policy, or ask someone who knows the office well. The word casual does not settle whether denim, trainers or sleeveless tops fit a particular setting. A day with visitors may also need a different choice from a day spent entirely with your usual team.
You do not have to copy a colleague's style to learn from the setting. Notice the level of formality: are people wearing relaxed shirts, dark jeans, simple knits or sportswear? Then choose a version that feels like your normal wardrobe. If you are new, changing one element of a familiar work outfit is a manageable starting point.
Relax one piece first
Swap tailored trousers for a clean pair of jeans while keeping a shirt and your usual shoes. Alternatively, keep the trousers and wear a soft knit instead of a jacket. This makes it easier to judge the result than replacing every part with something unfamiliar at once.
Pay attention to condition as well as garment type. A favourite jumper with stretched cuffs may look more informal than you intended. A fresh T-shirt under an open overshirt might feel perfectly suitable in one office and too relaxed in another. The workplace context matters more than a universal list of approved items.
Let the day's work decide the rest
If you will move equipment, travel between locations or stand for long periods, allow for those jobs. Test the clothes while bending, reaching and sitting. Keep an extra layer available if meeting rooms are cold. The aim is not to assemble a special Friday costume, but to make a small, practical variation on clothes that already let you do your work.
