Evolving weapons happen by combining a max level weapon with any level of a specific secondary support item. Once you have both of those items you must then kill an Elite enemy and open the chest that it drops. The combinations for the Garlic is the following:
Soul Eater – Garlic + Pummarola
Garlic is a weapon unlocked by first eating 5 floor chickens (found by destroying light sources) and Pummarola needs to either be discovered or first unlocked for general drops by surviving 5 minutes as Gennaro Belpaese (The barbarian with the knife)
If you're interested in seeing what the combo is for the other weapons they are listed below in the spoiler tag :)
*** Spoiler - click to reveal ***
Hellfire – Fire Wand + Spinach
Death Spiral – Axe + Candelabrador
Holy Wand – Magic Wand + Empty Tomb
Thousand Edge – Knife + Bracer
Heaven Sword – Cross + Clover
Unholy Vespers – King BIble + Spellbinder
Bloody Tear – Whip + Hollow Heart
La Borra – Santa Water+ Attractorb
NO FUTURE– Runetracer + Armor
Thunder Loop – Lightning Ring + Duplicator
Gorgeous Moon – Pentagram + Crown
Vicious Hunger – Gatti Amari + Stone Mask
Mannajja – Song of Mana + Skull O'Maniac
Valkyrie Turner – Shadow Pinion + Wings