There are a few detailed threads about it on the forum, but the basic message is put it in a pot on the lowest heat possible and leave it till the pieces of fat turn into hard crispy things,

I usually render tallow in the oven. I get about 2kg grass-fed beef fat from my butcher (I go in on a Monday, as I know they break down carcasses that day) then I chop it and put it in a big roasting pan in the oven at about 160C and keep pouring off the fat as it melts off over the next couple of hours. I pour it through a seive into a heat-proof bowl to cool a bit, then pour it into wide-mouthed glass jars for storage in the fridge. That gives me enough tallow for my family of five for a couple of months. If I was storing it for longer than three months, I'd wrap blocks in greaseproof paper and freeze it.