This is not really something that is a DIY. You will want to buy a premade grate like this. However in my humble opinion they are not effective in actually getting heat out of a wood burning fireplace.
If your goal is to get usable wood heat at a low Price tag.then save you nickels for a used fireplace insert or wood stove. Put word in with your local fireplace shop that you are looking for one.

What can or should I do or use to get more heat out of mire fireplace?
My fireplace is a wood burning fireplace. It's 2-2.5 ft deep and 3 to 3.5 ft wide. I've got a standard grate in to hold the wood for burning. I'd like to get more heat out of it. Seems it's only about 10% effective at best. I've looked at Fireplace Heaters (grates with blowers) and that might work. Cozy Grate seems to look like it might be the best. I've also thought about adding Iron plate around all 3 sides of the fireplace to help generate more heat. Think this would help? Anyway I'm currious if anyone knows what more I can do to get more heat outta this sucker. I'm payin bout 60$ a rick (cord) for wood and would like to garner as much a financial benefit from it as I can. Oh...and another thing..Can I add a wood burning stove to a standard chimney without any real modifications? As you might tell I'm a city boy...HELP ME?? Thoughts?? Brrrr...Cold here in Indiana

The problem is that when the fireplace is burning that it takes air from the room and it goes up the chimney, the air is replaced by pulling in outside cold air, so as MrNeutra... mentions a fireplace insert is the best, as it minimizes the hole that the air rushes out. If you cannot put in a fireplace insert, then you need something that blocks the hole when you are not using it and can closed for the most part when it is running. If you can get an outside source of air for the fireplace, then the radiant heat will heat the house and it will not suck air from the house. The grates can be helpful, but not if the opening is covered by only an open-weave screen. Something that sucks in the air at the bottom (coolest air in the house) and blows it out the top, after it is heated by the fireplace fire.

My old furnace, needed replacement and when my new one was installed, they had to change the old chimney as the building code had changed. They put in a steel liner, so do not assume that changes can be made to your existing fireplace without changes to your chimney. If the code has changed and you modify something, and it is not to code, then if there is a problem, you may find that your insurance is void.