Purchase a big folder that has a separate compartment for each subject. Label each compartment with a white sticker, on which you'll write the name of a different subject.
Make daily notes on what was accomplished in each subject. This will prove the consistency and progress of your child's work. Write down titles of all the reading materials the child used in order to accomplish her work, as well as the materials they used for extra credit. Include dates on these notes and always keep them filed in chronological order.
Add other materials you feels best document your child's work. You don't have to keep everything, but even if you do, it won't be wrong. Include as many worksheets, writing assignments, workbook pages and creative projects as you'd like. Put the materials in the space assigned to the subject they relate to. If something doesn't fit in the folder, take a photo and file that.
Keep the portfolio well-organized and update it regularly. Put dates on all the materials you put in the folder and keep them in chronological order. Research your state's regulations to find out how long you are legally obliged to keep the portfolio. It is usually two years after the completed school year, but it can differ from state to state.