Where are they too big?
You can stitch a band of wide elastic to the inside o f the waistband so the waist is made smaller without being seen (the strip of elastic must be shorter than the length of the waistband for this to work).
If the legs are too wide take them in on the INSIDE leg seams so the outside leg seams are untouched.
If they are a little too big, wash them in hot water and dry in the dryer, otherwise Fiddledd has some very helpful suggestions.
I like this tutorial for adding button hole elastic to help cinch in the waist of your jeans: https://www.youtube.com/watch?v=2xM27ufP3Ac
If they are too large in the hip, it will necessitate removing waistband and pleating, or disassembling jeans to recut to smaller proportion, over the hip area, then reassembling front to back, noting that waistband may require alteration, as well.
Jeans usually shrink an average of 3%, so try washing in hotter water, first. Dry on hotter temp, only if necessary, to avoid overshrinking.
As a former seamstress, Fiddledd suggestions are exactly how I would alter them