Portfolios of work are very common in certain fields, like photography, modeling, art, architecture and so on. They are good for school students as well as long as they are not too complicated and onerous to use.
The Council of Europe has a European Language Portfolio but this is over complicated. It has three parts: a language passport, a language biography, and a dossier. The language passport lists your proficiency in various languages. The language biography is a history of your language learning. And the dossier is the actual portfolio part, and that is what we are really interested in. The other information would be found on your CV for example so is extraneous.
A portfolio is simply a collection of works arranged in some way, perhaps chronologically, to show to someone. So, it’s basically a folder of some kind. It could be a real folder or a virtual, electronic folder. Of course, it’s best to have a paper version and an electronic one, just in case.
Keep it simple is my advice. Just a simple folder of A4 plastic sleeves. All you need to add is a cover page and a contents page. Scan the materials as well.
You can use this text to introduce the idea to your students. Let them keep the portfolio in the class and after they have written a work which could go in the portfolio, let them produce a clean, final copy and decide whether to add it or not.
Keep it simple. Keep it straightforward. Keep it easy.