The first thing you need is a fabric, preferably a muslin cloth. While any colour will do, historically white was the color of choice back in the day.
For men the cloth needs to be one and a half feet wide which is more than enough to cover the body, while for women it should be long enough to cover the chest and fall below the waist. Men can leave one shoulder uncovered if they want, for women both shoulders need to be covered.
Now fold the sheet. You may need to fold it twice.
After folding the sheet men need to hold it around the waist line while women need to hold it just under the armpit. You now need to wrap the sheet around yourself such that the end is held in place.
After wrapping the sheet around you, it is time to wrap the long end over the shoulder from the back.
Bundling the sheet over the shoulder, you need to tuck this bundle in the part of the sheet with which the waist/chest is wrapped.
Now inspect your toga to make sure the folds are neat and it will remain in its place.
Just to be on the safe side you can choose to pin the toga to keep it in place and prevent it from falling off.
You can also use a belt to hold the costume in place or just for the sake of completing the look. The belt needs to resemble a rope as according to Greek and Roman methodology, ropes were used as belts around the waist.
Always remember to keep checking the toga during the time you are wearing it to avoid any dress malfunction, as it can easily slip off.