You mean... pellets soaked in garlic juice? Or are you meaning to feed him a little bit of garlic?
For the pellets soaked in garlic juice, take a day or two's feeding of pellets. Get a few cloves of garlic and crush it up as well as you can, or use a garlic crusher (this is most effective.) Once the garlic is crushed, put the pellets in the garlic, and mix them around a bit. Then put it in the micro-wave for about 20-30 seconds.
For the garlic pieces... just crush up a garlic really really finely and feed a teeny bit to him. The garlic pieces contain garlic juice anyway.
For the garlic juice... moosh up some garlic and you might have to squeeze the moosh to get the jucie out. Never tried that.
By the way, it will take your bettas a day or two, usually, to get used to the pellets soaked in garlic juice or the garlic pieces. I've never gotten any of my bettas to actually eat the garlic pieces, but they have eaten the pellets soaked in garlic juice.