Basic voiced katakana
The following 25 katakana are derived from the basic unvoiced ones we just looked at: ga / gi / gu / ge / go are written exactly the same way as ka / ki / ku / ke / ko, for example, but with an extra two strokes in the top-right corner. The dashes denote a "voiced" sound: ga is the voiced counterpart to ka. (Try saying both, one after the other, and you should realise that they are effectively the same sound - rest your fingers lightly on your throat as you do so and you should be able to feel that the only difference is how far down your throat the sound starts.) So if you know the first batch of 46 katakana, these 25 should be relatively simple to pick up.

So ga and ka are a voiced / unvoiced pair, as are gi and ki, gu and ku, and so on. The same holds true for za and sa, zu and su, da and ta, etc.

It kind of breaks down a bit when you get to ji and shi, which aren't really a true phonetic pair. Ditto ji and chi, and also zu and tsu. But this is, nevertheless, how the Japanese alphabet works. Sorry.

Notice also the two blank spaces in the middle of the table, where you might have been expecting a and a . These would be pronounced ji and zu respectively, but these sounds are always written using the katakana and instead. The katakana and are so infrequently used that they might as well not exist. (The corresponding hiragana, and , are occasionally used - but only occasionally. Again, and are far more common.)

Notice also that adding two strokes to ha / hi / fu / he / ho gives us ba / bi / bu / be / bo , even though ba isn't really a voiced form of ha - but, again, that's how it's written in Japanese. Notice that we also get pa / pi / pu / pe / po formed from ha / hi / fu / he / ho by the addition of a small circle rather than two dashes.