1.First things first, make sure it's not raining or check the weather forecast to make sure it's not going to rain anytime soon. It's very dangerous to use outdoor electrical appliances in the rain.
If the weather is fine just give you trimmer a once over and make sure everything is in working order.
2. make sure the area you are going to trim is free from hazards. If you have pets or children just make sure they are at a safe distance and looked after.