Some things you should bear in mind:
Never double up. Wearing two condoms at the same time may seem safer, but it’s not. They’ll rub together and tear.
Never use a condom twice.
With anal sex you have to use a special condom that’s made for it. Don’t use a regular condom: the rubber is not strong enough.
Don’t use condoms which have passed the ‘best before’ date, which have been exposed to the sun for a long time or have been in your back pocket too long.
Don’t use fun-condoms for intercourse. They are good for a laugh, but not for safe sex.