Not a DIY. You need a contractor.
Hire a concrete contractor. Pouring is an art. Or if you want to DIY, rent a cement mixer for best results. When pouring concrete, prep is key to a flawless result. Concrete is also weather sensitive as it can't be too hot or too cold when you pour to eliminate surface cracks.
Hire a concrete contractor. Pouring concrete is an art. Also temperature sensitive. Can't be too hot or too cold. If you want to DIY rent a concrete mixer. Prep is key to a flawless result. Here is how.
2 by 4 s and frame it up first
I'm Linda and I'm going to agree with William and Naomi. If you want a perfect sidewalk then you should hire a contractor. The perfect sidewalk is hard to do as a DIY project if you don't have experience. Plus, depending on where you live, a perfect sidewalk may not stay perfect over the course of a few years. The earth moves and if your climate gets hot & cold, the sidewalk will expand and contract so any concrete/cement will develop cracks. Wishing you the best.
In addition to all of the very good advice posted above, be aware that if you live in a cold climate, you should consider installing underground piping that will send heated water beneath the concrete (or electric), to keep the concrete free of ice and snow. Although it may be costly, you will appreciate not having to shovel snow, chip ice, or worry about spalling concrete due to salt being used to melt ice.