You do not have to change the base forms of nouns at all, just add the interchangeable endings in the chart below. I have put casual forms, (used with your in-group such as family and friends), in green. Note that there are two alternatives for the negative conjugations. The first alternative ends in ~nai or ~nakatta. You can drop "desu" for casual style speech. The second alternative ends in ~masen or ~masen deshita. Both of these are appropriate in polite (teineigo) speech.