A lined suit jacket can be shortened, but you cannot just fold it under and sew it down without creating excess bulk. Folding alone would mean leaving a hemline four layers thick. If possible, have someone help you measure and mark the suit jacket. Making a straight, professional-looking hem will be faster and easier if someone helps you with the preparations. Otherwise you will be raising and lowering the jacket's hemline as you twist your arms around to set the back hemline marks.
Put the jacket on and have your helper mark the desired hemline at the centre back, and every 15 cm (6 inches) coming around the jacket from the back to the front. If you do not have a helper, set the first pin then let your arms hang naturally and see if the pin is where you want it. If it is not set in the desired spot, try again.
Remove the jacket and spread it out flat with the lining against the table's surface. Line up the top edge of the ruler or yardstick with the pins marking the jacket's new hemline. Draw a straight line along the ruler's bottom edge, 3.75 cm (1 1/2 inches) below the pins, using the tailor's chalk. Wait to remove the pins until later, after you have folded the hem.
Cut along the chalk line you drew on the bottom of the jacket. The chalk line will be 3.75 cm (1 1/2 inches) below the pins marking the desired fold line. Cut through both the lining and the jacket fabric.
Cut the threads holding the remaining hem stitches using the seam ripper. Hook the seam ripper's pointed tip under the thread and push forward and up to cut the thread. Open the seam holding the lining to the jacket's front edge, going 7.5 cm (3 inches) up from the bottom.
Measure 2.5 cm (1 inch) up from the lining's cut edge and mark every 15 cm (6 inches) with a tailor's chalk dot. Line up the ruler or yardstick's edge with the dots, and draw a line connecting the dots. Cut along the line using the scissors, cutting only the lining.
Finish the cut edges of the lining and the jacket fabric using a zigzag or blanket stitch on the sewing machine. Use a tight, stitch to prevent fraying.
Turn the jacket inside out and spread it out flat with the lining facing up. Smooth the lining, making sure it is wrinkle free. Fold the jacket fabric up and over, encasing the lining's edge, using the pins as a fold line. Remove each pin from the fold line as you go, and place the pin through the jacket fabric and the lining.
Fold the corners at the jacket's front hemline to match the way they were folded originally. Pin the folds down with straight pins. Look at the piece you cut off to see how it was folded if you need a reminder.
Hem the shortened jacket by hand-sewing using a catch stitch. Anchor the thread by making two 0.16 cm (1/16 inch) stitches in the lining just under the edge of the jacket fabric, at the beginning of the hemline's opening. You only have to make these two anchor stitches if you do not have a knot in the thread. If the thread is knotted, go on with making the hem.
Move the tip of the needle about 0.6 cm (1/4 inch) forward, to the right. Point the needle's tip to the left and pick up two or three fibres of jacket fabric. Draw the thread tight, but not tight enough to make it wrinkle. Move the needle tip another 0.6 cm (1/4 inch) to the right, point the tip to the left and pick up two fibres of lining fabric. Continue all the way across. Anchor the stitches at the other end by making two or three 0.16 cm (1/16 inch) stitches.
Iron, using a lukewarm iron, to finish the hem.
The lining might be cut with a certain amount of ease, or excess fabric, so it does not fit tightly inside the jacket fabric. This might cause places where the lining has a puffy or wrinkled look along the hemline. This is normal and you can smooth it down with your fingertips and go on with hemming.