How do I place an newspaper ad to divorce my husband when all I know is that he's in Texas?

Well first you have to file for divorce in the proper court, you will then address this with the judge. Your local court will have its specific rules as to which papers they requrie. Normally it will be a certain local paper ( in legal section) and also in the area he was last known to live at.

There are requirements to attempt to find him, such as contact his relastions, his parents, brother or sister and so on.

You really need to get an attorney invovled on this

You need a lawyer because if this is messed up (invalid) and you get your divorce, and remarry, the marriage will be invalid as well.