Salaam alaykum or Peace be upon you. Wearing Hijab in the workplace needs to be done from the day you start your job. Otherwise it may create an issue with the employer if you suddenly change your dress. At the very least, it will be uncomfortable to walk into work in hijab after you have been there without it. But this can be done and other Muslim sisters have taken this step.
If your work requires a uniform with short sleeves then you must purchase a black long sleeve undershirt to wear under the uniform. And wear long loose pants or a full length skirt. Black, red, navy or gray are the most professional colors for this. Then match your hijab to your undershirt by wearing a black plain hijab that’s not loose and does not flow, for safety reasons. A two piece al amira hijab works best because they are quiet fitted and snug. They can not get caught up in machines if you work in a factory, or fall into foods if you work in food service.
The hijab at work will present some issues to others. For example, you might get other coworkers starting to wear inappropriate things like ball caps and hoodies at work because they wrongly assume you are taking liberties at work. This is why its important for you to confront your boss yourself on the first day in private and explain to him that you are Muslim and have changed your appearance for religious reasons only. In most countries freedom of religion is respected and your boss has to allow you certain religious accommodations. But if you do not confront your boss in a friendly, proactive way then you may end up with some negative feedback from other coworkers and some drama in the workplace.
There are plenty of youtube users who make hijab tutorials for work and you may like to browse youtube for instructions on how to wear hijab at work. They give different styles for different people and there are plenty to choose from. Some popular videos are: sister amenakin, sister Zukreat, and user alz786uk.
Remember, that hijab is part of Islam and if someone loses her job for putting on hijab (which should be reported to the national human rights tribunal in their country) then it was part of Allah’s plan. Do not be sad because whatever losses occur for obeying Allah, will be rewarded with a much greater blessing in future. Maybe giving up a bad job because of hijab will bring a person to Jannah/heaven, Allah knows.