DISCLAIMER: I am not a CFI! I am, however, an instructor Flight Engineer with over 5000 hrs and 14 years in Airforce multiengine jets. As an FE one of my primary duties and my bread and butter is performance data so I take it very seriously.
To calculate DA you need your Outside Air Temp and Pressure Altitude. Here's a quick step by step guide to get the numbers you need.
First of all you need an altimeter setting and field elevation to calculate your Pressure Altitude.
Pressure Altitude is defined simply as Altitude corrected for non-standard pressure. At 29.92 (standard pressure) your altimeter will read field elevation. Since it's hardly ever standard we have a chart to correct for that. Remember the old adage high to low look out below? Well here's where that comes from. Set your altimeter to read field elevation. Once you have that reading come to the following chart.
Click to make it larger
Let's say for example you are sitting at Johnson Creek and your altimeter reads 30.13 to get the field elevation of 4933 feet. To get your Pressure Altitude go into this chart from the left side at 30.1 and go right until you hit the column that reads .03 on the top as you can see the correction number is -193 feet. This is only a correction NOT YOUR PA!
Subtract 193 from 4933 and you can see your pressure altitude is 4740ft. Looking at the chart you can grasp that anything above 29.92 lowers your PA and anything below that raises your PA. Another general statement is every .1" equals roughly 100ft. So 30.00 to 30.10 lowers your PA by approx 100ft.
Now that you have your PA let's figure out Density Altitude.
Density Altitude is defined out of the Mountain Flying guide as:
Density altitude is a term that sometimes causes confusion to the uninitiated. A high density altitude is NOT a good thing. Density altitude is defined as the pressure altitude corrected for non-standard temperature variations. And while this is a correct definition, my definition is perhaps more appropriate: DENSITY ALTITUDE IS THE ALTITUDE THE AIRPLANE THINKS IT IS AT, AND PERFORMS IN ACCORDANCE WITH THIS COMPUTED VALUE.
To compute your DA use the following chart
Start at the bottom of the chart with your temp. I have a cheap digital walmart temp gauge with a sensor stuck to the bottom of my wing in my airplane just for this data.
Go straight up until you hit your Pressure Altitude NOT your Field Elevation.
Read left to get your Density Altitude. As you can see temps upwards of 35 Celsius which is 95F would take the Density Altitude in the before mentioned example at Johnson Creek to over 8000ft. Temps upwards of 90 degrees during the summer months are NOT uncommon in the Idaho backcountry during the heat of the day.
The dotted line from the bottom to the top is the 15 degree line. If you remember from way back during your training 15 degrees is standard temp. 15C is only 59 degrees F folks. Anything over that and your performance is starting to decrease on top of however high you already are.
In case you think this is just a bunch of mumbo jumbo that doesn't apply watch this video again.
http://www.liveleak.com/view?i=835_1344412426
Hopefully this is clear as mud. Feel free to contact me via PM if you want clarification on anything.
