Easy. Just use electro-magnetic fields to keep matter and antimatter plasma apart. Switch off the field when you want the big boom.
The difficulty is getting the antimatter. Given that it's rare in our bit of the universe and the only other (man made) source requires vast amounts of energy and time as its made in miniscule quantities.
Given the way the modern world is, I've probably said enough to have a knock on the door and disappear as some kind of terrorist now ... because having knowledge is the same as doing these days, apparently. Didn't Pol Pot do similar in his country?