How do you replace a heater core in a 1986 ford tempo?
The complexity of a heater core repair is such that it doesn't lend itself to describing here. I'd suggest purchasing a Haynes manual for your vehicle -- it's essential for a job like this, and will only cost around $16.
Search Ford Tempo Questions
Are you a UK consumer? CarGurus now has a discussion forum in the UK.
Ford Tempo Experts