The two most important considerations in setting up your rain barrel are a stable base and proper overflow control. Water is very heavy, over 8 pounds per gallon. A full 55-gallon barrel can weigh over 450 pounds! It’s important that you level the ground under your barrel and create a secure base using a material such as concrete blocks.
Your barrel should come with an overflow outlet installed near the top. This is where excess water will escape should your barrel start to overflow. The simplest method to deal with overflow is to attach a long hose or tubing to the overflow outlet and direct it well away from the foundation of your home. Another option is to use a short piece of tube and connect multiple barrels together. This is called "chaining" and increases the amount of your rainwater storage while decreasing the chance of overflows.
Once you have a secure base, determine how high up you will need to cut the downspout (most downspouts can easily be cut with a hacksaw). Once cut, you can leave as is, or add a flexible gutter piece to direct flow to the barrel opening.