Reloading can be a safe and enjoyable hobby as long as you obey some simple rules. Just as you follow basic gun handling rules to make the hobby safer, you must so the same with reloading.
Never mix or substitute components.
Every lot of powder, primers, bullets, brass, and any other components can differ significantly. Different manufacturer's components differ substantially. For example, never substitute Federal brand primers for Magtech, and expect the same results. One primer may be fine with your load, but another brand may cause extreme pressures with the same load. Don't substitute magnum primers for standard primers for the same reason. Just because two brands of powder have similar characteristics, that does not mean they are interchangeable or mixable. Every time you finish with a lot of a component, you will have to back off the load slightly and start over, checking for overpressure signs, just as you did when you first worked the load up. You are in essence working up a brand new load.
Always wear eye protection.
You hopefully wear ear & eye protection while shooting, and you should do the same while loading. You don't need the ear protection (hopefully) while loading, but eye protection is an absolute. Besides the obvious protection against an accidentally detonated component, you are protecting against flying particles (i.e. a piece of brass that jammed and shot out from the press). This is a press, and presses can generate some large forces. You are also working with hazardous components, mostly lead. You will be less inclined to wipe your eyes with your lead stained hands if you have glasses on.
Never eat, smoke or drink while working.
Just as your hands can introduce lead into the body by rubbing your eyes, food can do the same. Nothing is worse than getting a lead shaving in your tuna sandwich. The smoking part should be obvious. You are working with flammables and explosives! If you must take a food, smoke or drink break, wash your hands thoroughly to remove any contamination and take your break away from your workbench.
Block out all distractions.
While working, block any distractions. This means TV, radio, wife/husband, kids, dog/cat, pesky neighbors, and so on. It only takes one second of lost attention to produce a dangerous load. While loading, you must give 100% of your attention to what you are doing.
Keep your workbench clean.
Keep a tidy workspace. This will make things go much smoother. You are less likely to run into problems. It is not hard to have your scale give you a false reading because it is pushed up against a stack of papers that should not be there. Immediately clean up any spills. Use a dust brush and pan instead of a vacuum because of fire/explosion hazards.
Keep all components in their original container, and stored properly.
Do not store primers, powders, or other components in anything but their original containers. You need the container for proper identification. The factory containers are designed for long term storage, and is the safest and best way to keep the components. Always read the warning labels, and follow the recommended storage method (usually in a cool dry place).
Keep good records.
Don't rely on memory, or a scribbled on post-it note for your records. Keep a good notebook, and track all lot numbers, brands, depths, weights, or any other data you would need to look at to go back and trace a problem, or reproduce a load.
Keep out of the reach of children and pets.
You don't want unsupervised children, or irresponsible adults near your equipment or components. It is easy for them to, at best, change your settings or spill something, or at worst, start a fire or mix your components up causing you to make a dangerous load. Don't rule out teenagers. Many young adults have a fascination with fire, and would just love to get a hold of a pound of gunpowder.
If you are in doubt of something, don't guess. Stop and get help. Call the manufacturer for assistance. Most good component producers have a technical staff that is eager to help. They don't want anyone to get hurt with their product.
Establish a good routine, and follow it exactly.
You will hopefully develop a method where you will have your own little production line. Once you find a good routine, stick with it. You will less likely have errors if you follow it.
Always check for overpressure signs while shooting your loads.
If a load seems strange, stop shooting it. Look for primer flattening or flow back. Also, case bulging, or difficulty with extraction. These are good indicators that your loads are too hot. Stop shooting them immediately and step your loads down. If recoil is severe (more so than with similar factory loads), then stop shooting them. A chronograph is a nice way to keep track of velocity. If the shots are significantly faster than what you were working for, you may have overpressure loads.
You can have low pressure loads too. If you get a mild pop instead of the usual BANG, then stop shooting them, and check your barrel for a stuck bullet. Never try to shoot out a bullet. It will ruin your barrel or gun, and possibly cause severe injury to the shooter or bystanders! This does not always happen from a load with no powder.
Following these basic rules will help you load safely, and get the most from your hobby. By no means is this a complete list. Read loading manuals, and keep an eye out for any others cautions. Above all, use common sense and good judgment.
REMEMBER, YOU ARE RESPONSIBLE FOR THE SAFE OPERATION AND STORAGE OF YOUR FIREARMS!