This is the most important part so pay attention! Seriously…

1. DIAL 911!
2. Start chest compressions as follows

The procedure to perform chest compressions is:

• kneel by the side of the victim
• place the heel of one hand in the centre of the victim’s chest (the lower half of the sternum, or breastbone)
• now place the heel of your other hand on top of the first. Interlock your fingers together
• keep your elbows straight and bring your bodyweight over your hands to make it easier to press down vertically
• press down firmly and quickly to achieve a downwards movement of 4 to 5cm (2 inches), then relax and repeat the compression. 

After each compression, release all the pressure on the chest without losing contact between the hands and the sternum

aim for a rate of compression of about 100 per minute. You can help your timing and counting by saying out loud ‘one and two and three and four…’ etc. If by chance you know the song “Stayin’ Alive” by the Bee Gees, you can do check compressions to the beat of “Ah, ha, ha, ha, stayin’ alive, stayin’ alive” as it would give you just about 100 beats per minute.

As a reminder, the compressions should be forceful—you need to get that blood moving!! Better too hard than too soft. Too soft and a person may not make it. Too hard and the worst thing is you could break a bone… better a broken bone than a lost life!

Sources from https://www.cprcertified.com/blog/right-depth-of-chest-compressions-in-cpr and from http://www.netdoctor.co.uk/health_advice/facts/heartattack.htm