Ok, this is just my bias, but I prefer to reauthor when combining dvds. This will let you make a menu where you can select each one individually. Or you can hit the Play All button if you want to play them in sequence.
I'll go through the process step by step for you.
First, download Vob2Mpg. I gave you the link earlier. Install it and start the program. Now, look at this picture:
Vob2Mpg normally starts in IFO mode, so you will be looking for the Video_TS.IFO file in your first dvd. Don't worry, Vob2Mpg will pick it out if you go to the Video_TS folder. Then start the extraction process.
Do this for each dvd, making a mpeg file in an output folder on your hard drive.
Now, download and install AVStoDVD from the link I gave you.
When you start AVStoDVD, you'll want to look at the main window and click on Preferences. Start with the Video tab:
And here is the Audio Tab:
Save the preferences as default and click ok.
Back on the main screen, add your videos and set output size (single or double layer dvd).