Remember the actor James Caan? That would be very close to "ka"
Say "key" but instead of elongating the "e", make it a short, snappy "i" -- "ki"
Same thing with "coo". Make the "u" much shorter to get "ku"
Say "Kay" (which in Romaji would be "Kei"), but instead of fully voicing "ay" stop at where it sounds like the Spanish "Que?" to get "ke" sound
Now the tough one, "ko". Best I could suggest is to say the Mexican snack "Taco" and make the "o" very short.
Otherwise, you just have to keep practicing with a native speaker.
Ganbatte kudasai.