It's just a drape that's pinned and/or tied. Most people just use a sheet. I found a link that might help- or it may be more than you want to know.

Here is a link that might be useful: how to make a toga

The beginning part about the men's toga on the previous poster's link is quite good. The picture of the lady in the peach coloured dress is good too. Remember, only loose women wore a toga as that is really a men-only garment. Also men wore a tunic (like a long t-shirt made of non-stretch material) under the toga.
To make the woman's dress, sew two long rectangles of fabric together up the sides, leaving a gap at the top of each side for an armhole. Sew the tops of the shoulder seams, leaving a space for your head (hem around the space). Alternatively, join shoulder seams together with a large, jewel-like button every 4"-6" so there are little gaps all the way up each arm, or just join on each shoulder and leave the excess to drape down at each side. Belt at the waist and pouch a little. Wear with a "stola", which is just what it sounds like, a long stole.
Women's clothing should be floor-length and the hair is worn up.
Men's clothing is generally knee length or longer.
To make the man's tunic, cut two pieces of fabric like a "T" with a wide middle (where your body goes) and a short cross piece (for the sleeves). It's best to pin this together on the wearer before you sew it up, to ensure it will fit. Sew together along the side and under-sleeve seams, and along the shoulder seams, leaving a hole for the head. Hem the hole.

You can add trim on any of these.
There was a strict hierarchy of colour to have each toga (different colour for ordinary citizens, nobility and priests) but I doubt it would be relevant to any but a purist.