This will describing how you blend out weld on a flat surface. The first step is to have a good weld; good Fit up (parts need to be aligned perfectly before welding, no porosity (gas bubbles in your weld), good penetration (so you don't grind off all the material holding your parts together), and no undercuts (the edges of your welds don't have groves or channels cut below the surface of the base material, from too much heat or not enough filler metal).
Always wear your safety gear, keep your parts clamped down securely, and keep the guard on the machine.
The basic premise is to start with your coarse grits, and only hit the high spots initially. You don't want to over grind the area on either side of your weld, and you don't want to grind grooves and deep scratches in your part. So keep the angle grinder moving, keep the grinding angle low, and keep crossing the direction of each swipe of the grinder.