I want to put a decal on a drinking glass but how do I protect it from

Don't put in the dishwasher and don't soak it in dishwater. Carefully was by hand.

What sort of decal? Some can be baked on.

I work with ceramics. There is a clear coat that we use to coat our finished pieces. You might want to try that. It goes on white but dries clear. Check with A.C.Moore.

What kind of material are you using? If you have a silhouette cameo or circuit you can cut Oracal 651 vinyl and it is permanent and dishwasher safe...with no sealer! I do it all of the time! I order my vinyl on Amazon

When we did this we used decoupage over it. It can be purchased at any Hobby Lobby or craft section of most department stores like Walmart

You can get Etching Paste to make an image on it. It's washable once dry.