Open the Door
The first thing that you need to do is to open the door of the garage. You can close it if you are uncomfortable with it being open when you work but keep it open for a while before you start working so that some air can move in and out to cool the area down.
Often there are appliances that are placed in the garage such as a fridge which can generate quite a bit of heat. In order to cool down the garage, it is important to move these appliances out to allow it to cool down.
Generally the garage has nothing on top of it so it is directly exposed to the sunlight which can make it hot quicker than the rest of the house. There are several treatments that can be done in order to cool it down. One of the basic treatments is to coat the outer part of the roof with white paint. The heat absorption will automatically reduce. Similarly, you can also get insulating tiles installed on the outer part of the roof to stop the heat from entering the garage. It will cost a touch more but it is a long term solution.
A shade for the garage can also reduce the amount of heat inside. The best way to go about it is to have a tree cover the garage. Obviously it is something that needs to be done when the home is being made. In case you do not have any trees, you can plant one which should grow tall in a couple of years to provide shade. You can also transplant a tree for this purpose.