Guitar strings need some downward pressure over the nut to aid string tension, tuning, etc. On some guitars, that pressure is achieved with an angled headstock. On other guitars, such as those with "flat" or "parallel" heads e.g. Fender, that pressure is achieved with string trees. Depending on your design, the tuning machines closest to the nut usually have sufficient pressure, but trees are needed for the remaining strings, usually the "high 4" strings. There are several designs for string trees, so look around on guitar supplier sites and find something that fits your design.