Listen to German soundbites or watch German films to understand how the language is spoken and how it differs from English. German often sounds faster and more harsh to English speakers.
Practice replacing the letter "w" with the letter "v." The German language does not use "w" the same way English does. Any time you use the letter "w," pronounce it like a "v." For example, if you say "I walked to the water" in a German accent, pronounce it "I valked to the vater." In another quirk of German consonants, "v" is pronounced like "f"; practice accordingly. For example, "Victor" is pronounced "Fictor" with a German accent.
Replace any "th" sounds with the letter "d" when speaking with a German accent. The German language does not use the "th" sound and German speakers have some difficulty making it. For example, "They are the neighbors" sounds like "Dey are de neighbors." Also, when you see a "d" pronounce it like a "t" instead. For example, pronounce "drink" like "trink" to sound German. The verb "to drink" in German is "trinken."
Sporadically pepper your speech with some German phrases and words to add a bit of flavor and a layer of believability. For example, use "Ich" instead of "I" on occasion. "Ich" is pronounced gutturally, in the back of the throat, and sounds like "ick" with a slight puff of air at the end.