actually, i dont really know if what ur sewing is a pillow case (u can take it off when soiled and have it washed) or ur referring to the pillow itself where u put in the stuffing.
1) if its the pillow case ur referring, never a seam is on the outside if the opening is at the center of the back cover. u cut two rectangular shapes, the other one (supposed to be the back) should be longer than the other one (about 4 inches, perhaps). then u get the longer piece and cut it into two in such a way that the 1st piece is 3/4 in length while the other one is, of course, 1/4. then u fold and sew one end of both the 3/4 and 1/4 size. u get the other big rectangular piece (supposed to be the front of the pillow case) and with ends lay it flat on a table, then with the 3/4 and 1/4 pieces of cloth (supposed to be the back portion of the pillow case), lay it over the big whole rectangular piece of cloth. remember, the 3/4 and 1/4 pieces should overlap each other as these are bigger than the front rectangular piece (remember that the right sides of both the whole rectangular piece and the two 3/4 and 1/4 overlapping pieces are facing each other). the wrong side should be outside. with the wrong sides facing outside for the front and back rectangular pieces, proceed with sewing all sides. after sewing, u can just turn it inside out. the sides with the overlap portion may be hand-sewed with something to put the flaps in place.
2) if it is the pillow itself that you are referring, u sew all 3 sides (wrong side of the cloth outside, of course) of the cloth. going on the 4th side, u can sew the side leaving a small portion, enough for your hand to get inside to place the stuffing, open and unsewed. then u turn the cloth inside out, u are ready to put the stuffing in. when u think enough stuffing is placed, u can sew the remaining slit manually, this time making "magic" stitches to leave it clean.
try it! i hope i can demonstrate it to you in actuality.