If you use liquid benedryl, mix it in with some yogurt and they should lick it all up just fine...
If you use benedryl tablets, you can coat it with peanut butter, stand behind the dog with your legs straddling their sides, use your clean hand to open up their mouths, then put in the pill/PB into the back of their throats quickly, close up the mouth and stroke their throats until it's swallowed...
We do this all the time, with our flee pills and any other medications...