An excellent tutorial on how to sew hems on jeans is available here.
I would like to add several suggestions.
In this photo regular default stitching is on the left, slightly elongated stitching is in the center, and triple stitching is shown on the right.
Denim needles or at the very least 90/14 needles are a must. Denim is a tightly woven fabric and hard to sew through with lightweight needles. When you reach very dense areas, such as, the multiple layers found at the seams you will need to slow down. Rotating the handwheel of your machine to pierce the fabric will save your sewing machine’s motor, your needles, and your nerves. You may need to advance the machine stitch by stitch across this very dense area. After you pass it you can continue sewing as normal.
Denim needles are also available as double needles. The caveat about slowing down goes double for these needles. :):)
Thread color is easily matched to the topstitching thread on the existing pants. But beware. Each pair of jeans is slightly different. Even the same manufacturer can have slightly different thread colors. Just check it out with each new pair of jeans. Normal sewing thread is just fine to use.
Select a slightly longer stitch length than the normal default. This allows you to more closely approximate the existing stitching.
Even better, use the triple stitch function on your machine. Most machines have this stitch under “utility stitches”. It also can be elongated slightly. This combination of matching thread color + elongated stitch length + utility stitch makes a very effective hemstitch on your altered jeans.
Finally, sewing over thick seams is always a problem. If the presser foot is not kept parallel to the bed of the machine you can risk breaking a needle. I can verify this by experience all too well. 🙁 There is a little accessory called a Button Clearance Foot. Look for Husqvarna Part No. 41 11 732 01 or 41 31 056 01. Most sewing machine stores can get this for you. It slips under the presser foot before or after it reaches the seam and raises the foot. This allows the machine to continue sewing safely and with the same stitch length. It’s one of the handiest little tools in my sewing drawer.
View the tutorial I mentioned here and consider the additional comments I have posted. In no time you will turn out hemmed jeans that even the fussiest person will be happy to wear.