Let us assume you need to design an alignment from point A to point B having a min grade of 1% and a max grade of 5% and let’s say pipe segments come in 20m lengths.
Issue the TLGradeDiagram command, select your OG surface and input the min/max grade parameters and the segment length.
Start laying out grade diagrams from one of the initial points, working your way to the next. The geometry generated by the grade diagram will outline the possible starting location for the next one. You can adjust the parameters as you go along if you run into a situation where there is no solution (terrain too steep) or too many solutions (terrain too flat). Once you arrive at the second point after chaining the diagrams in this fashion simply draw a polyline through all of them and you'll have the first draft of an alignment meeting the design criteria. You may go back and explore different branches and/or different options.

Parameters:
Surface: Existing Civil 3D Tin Surface object - original ground surface
MIngrade: minimum percent grade, double, positive number or zero
MAxgrade: maximum percent grade, double, positive number, must be equal or bigger than min grade
Distance: segment/leg length, a double, positive number

Additional parameters:
Points: Number of diagram points, integer, a positive number between 10 and 3600. Determines how many points are sampled around the start point and how many vertices the diagram envelope will have
SCale: height difference scale factor (k), double, no range limitation, refer to envelope definition and calc diagram for details

Color-coding:
White: Directions with grades less than the min grade in absolute value are shown in white color
Green: Downhill directions with grades less than the max grade but more than the min grade in absolute value are shown in shades of green
Red: Uphill directions with grades less than the max grade but more than the min grade in absolute value are shown in shades of red

Diagram envelope
The radial lines of the grade diagram visualize the possible direction vectors meeting the criteria of the preset grade. Lines having grades bigger than the max grade in absolute value are not drawn indicating the grades in those directions are too steep. To provide the designer with some sense of how far off those grades are a grade envelope is drawn around the start point. The envelope is meant to be a metric providing a 360-degree view of the steepness of the terrain. Refer to the envelope calc diagram for technical information on how the exact location of points on the diagram is computed.
The diagram envelope has the following general properties:

• A perfect circle with a radius of d around the start point indicates a flat OG surface
• Steep downhill (negative) grades push the envelope away from the center point by as much as twice the distance as the grades approach negative infinity
• Steep uphill (positive) grades pull the envelope towards the center point as the grades approach positive infinity
• An envelope scale factor of zero will flatten the grades and will result in a perfect circle envelope
• A negative envelope scale factor will reverse the downhill/uphill behavior described above

The installer that ran when you downloaded this app/plug-in from Autodesk App Store will start installing the app/plug-in.

OR, simply double-click the downloaded installer to install the app/plugin. 

You may need to restart the Autodesk product to activate the app/plug-in. To uninstall this plug-in, exit the Autodesk product if you are currently running it, simply rerun the installer, and select the "Uninstall" button. OR, click Control Panel > Programs > Programs and Features (Windows 7/8.1/10/11) and uninstall as you would any other application from your system.

Once the installation is complete, start Civil 3D and login with your Autodesk® user id and password. Run the TLGradeDiagramActivate command to activate the app. Once activated the app can be used offline.