Follow the manufacturer's instructions. Dont allow the soution to contact bare skin. Use forceps or an old plastic clothes-peg to handle the PCB. Wash your hands thoroughly afterwards.
The most common fluid used for etching is made by dissolving Ferric Chloride crystals in water. It's available in crystal form or ready-dissolved. Like all chemicals, it needs to be handled with exteme care.

It is toxic, corrosive and will badly stain anything and everything it comes into contact with. This entire operation is best performed in the garden shed or garage - well away from children and pets.

Pour sufficient solution into a suitable plastic or glass dish - I use an old photographic developing dish - and immerse the printed circuit board.

The instructions accompanying the etching solution will indicate how long the etching process will take but the solution can be used over and over again and the etching time increases after each use. Fresh solution should take about 40 minutes to an hour.

Lift the board occasionally to check on its progress and to ensure fresh solution is on the board's surface. Don't leave the board longer than necessary, otherwise the solution will begin to 'undercut' the etch-resist.