You were on the right track when you were thinking of taking in the inside seam. Its really not that difficult. I will do my best to explain but you may be able to find a tutorial also.
Put on a pair of pants that you can "play around with". Have a friend you trust do the pinning.
1. On the center back crotch seam (also called rise) "pinch" the fabric horizontal
until it feels comfortable. This could be as little as 1/2 inch to a few inches. Pin the amount pinched.
2. After you take off the pants, mark and measure on the inside, the amount pinched on the center back seam. This is the measurement you will use to take in the inseam, but only the back of the leg.
3. Say the amount measured is 1 1/2". You will need to open both the inseam and the crotch seam. Measuring from the top of the inseam on the back portion of the leg measure 1 1/2" from the seam and make a mark. Taper this line to flow back into the original inseam. This is your new seam line for the back of the leg. The front of the leg seam with remain the same. You may need to take it to just the knee or all the way to the hem. You will be taking out a long V shape.
4. After you have re-sewn the inseam, re-sew the U of the rise (crotch). Most of the time you can follow the original seam, in extreme cases you may need to slightly re-curve the center back seam.
1. Pinned to take up seat
1. Close up view ~ Pinned to take up seat
2. Inside view
2. Close up ~ Inside view
3. Taper back into original inseam
In some extreme cases you may need to mark both the front and the back center crotch seams. If the front seems to sag and the back fits comfortably then adjust your horizontal pins. Remember to leave enough ease so the pants are comfortable while sitting and walking.
Once you do this alteration once it will be a breeze. The first time is a little intimidating if you have never done anything like this before but it is one of easiest things to do. Taking the seams apart is the most time consuming part of it.
I've updated this with a drawing, no laughing at my artistic skills
This type of alteration is done more often on men's slacks than women's but you can find detailed instructions in Mary Roehr's books. I'm sure there are other books out there but her's are very affordable and easy to understand.http://www.maryroehr.com/index.html