There are several historical references to the subarmalis, a garment worn under the armour to protect clothing and body from chafing and soiling. A 4th century description, which uses the term thoracomachus, says it is made of thick cloth, covered with leather (or with a separate leather garment over it) for waterproofing. However, there are no archeological remains or certain depictions.
One important function of the subarmalis is to give the shoulders padding against the weight and abuse of the armour. M.C. Bishop points out that shoulder pads also raise the collar plates up to the narrower part of the neck, alleviating "neck pinch", and causes the breastplates to hang straight, not at an angle. (Original breastplates always have their straps mounted vertically and horizontally, but do not show evidence of the angled stress suffered by modern reconstructionists.)
The exact form of your subarmalis is up to you. Padding for the shoulders can be as simple as a 12" to 18" square of sheepskin with a headhole cut in the center, but a more complete garment is recommended. It should fit fairly closely and be more or less hidden by your armor. It may be a pullover like a tunic, or have an opening at the front or side. It may be of linen, wool, or leather, and can be padded at the shoulders or all over. Pteruges (rows of flaps) may be attached at the shoulders and hem if you wear mail or scale armor, but they seem not to have been worn with the lorica segmentata very often.
A subarmalis with a linen lining and an outer layer of wool, heavy linen, or leather can be made as follows. Start with a short linen "tunic", a little longer than your lorica and just wide enough to get into, sleeveless with a narrow neckhole. (Since it is a lining it is made "inside out"; with finished side towards your body.) Pin folded scraps of fabric to the shoulders and adjust them to the desired placement and thickness, trying on your armor for a good fit. (If you have no armour yet, the length should be a few inches longer than your waist for a lorica segmentata, or about knee-length for a hamata. The shoulder padding should be approximately 1/2" to 1" thick.) Stitch the padding in place. Make the outer covering a little wider and a couple of inches longer than the lining. Put the layers together and secure at the neckhole by turning in the edges and stitching. Quilting the padding in place is also a good idea.
Now put on your subarmalis and your armour, and make sure the subarmalis is not bunched up or crooked. Trace around the bottom edge of the lorica, cut off any excess fabric, and hem. Also mark and trim at the armholes so that there is only enough subarmalis projecting beyond the metal at the armpit to protect the tunic, and hem that, too.