I'm confused. What is the exact problem you're having? Dakuten mark a sound as voiced rather than voiceless. So the sounds are formed exactly the same way except they are the voiced versions.
Example: γ is ka, γ is ga. Both of these sounds are produced with the tongue in the same location in the mouth, the only difference is that the "G" sound is voiced (meaning that your vocal chords vibrate when making the sound).
So it's all about remembering the sound of the character without dakuten and then just voicing that sound (okay, the "H"s are a bit different as the "H", and "B" sounds aren't articulated in exactly the same way ["B" is a voiced bilabial sound whereas "H" is a voiceless glottal fricative.]).
So...k becomes g, s becomes z, t becomes d. The only exceptions are that both γ and γ’ are pronounced ji, and both γ and γ₯ are pronounced zu.