Use a traffic free area with firm ground like a tennis court, playground or grassy park.
To start, you need to learn to balance, so have your saddle low enough so you can place both your feet flat on the ground whilst sat on the saddle. Then use your legs to push yourself along - like the original hobby horse riders did, a bit like walking astride the bike. It can help if someone takes your pedals off initially to prevent you banging your legs.
To ensure that you are confident at this stage, practise using your brakes - gently squeeze them with both hands at the same time. You can, if it helps, leave your fingers over the brake levers even when not pulling the brakes, but ready to use them whenever you feel you need to.
Take it slowly at first and build your speed up as you gain confidence. This builds your skills in balancing the bike whilst moving along, using the handlebars to steer and hold yourself upright.
For very short sections, see if you can freewheel with both your feet just off the ground. Build this up so that you feel confident to ride along and steer to where you want to go (without making sharp turns, obviously!) and to slow down using your brakes. A very gentle slope can help here.
Now you can balance, add in the pedals. Start with your preferred foot on a pedal at about the two o'clock position, push off with your other leg and press the pedal. As you are moving, put your other foot on its pedal, and you're off! Try to keep the handlebars straight but don't lean on them, look ahead not down and keep your back as straight as possible.