Install the Addon through the preferences window. The addon appears as a tab on the N Panel of the 3D viewport.

Select "Draw Flames" to draw flame curves 

Select "Add Flames" to add flames to the selected mesh object. There must be a mesh object selected for this to work properly. 

Note: Flames rely on displacement from object coordinates, so object scale effects the result. Adjust and apply the object scale to help control the flame appearance. 

Flames point in +Z direction, so object axis must be orientated accordingly. 

Works with EEVEE and Cycles. If using Cycles, increase "transparent" value in "light bounces" for the rendering properties, or else the materials will not display properly. Try a value of 20, and increase from there as needed.  

You must finalize the current object before adding flame to a new object. Press the "Finalize" button at the top of the panel. If you wish to change settings after finalizing the object, you can find all of the settings available in the modifiers window for the object. 

When rendering an animation, if including sparks or volumetric smoke in your flames, it is recommended to bake the simulation nodes by pressing the "Bake" button in the physics panel. 

Settings:

• Flame Coverage - how much the flames cover the object. By default they appear from upward-facing faces. Change this value to increase/decrease the selection where flames are created.
• Flame Height - the height of the flames.
• Subdivision Level - how many subdivisions to apply to the flame. 
• Flame Scale -  some control on the scale of displacement pattern. Effected by object scale.
• Texture Scale - some control on the scale of texture noise pattern. Effected by object scale.
• BURN PATCH
• Burn Size - effect the size of the burn patch material
• Use Vertex Weight Mask - use vertex groups for flame placement instead of default. When using this option, a vertex group is automatically applied. Use weight painting tools to apply flame where desired. When this option is used, Grow Selection option appears. This option expands the painted weight vertex group. 
• SPARKS
• Particle Density - amount of spark particles
• Lifetime - number of frames spark exists for
• Size - spark size
• Speed - speed sparks rise at. Combine with lifetime to set maximum height. 
• Vorticity - how much random motion added to spark movement
• SMOKE
• Particle Density - Viewport - number of smoke particles in viewport
• Particle Density - Render - number of smoke particles in render
• Lifetime - number of frames smoke particles exists for
• Speed - speed smoke rises at. Combine with lifetime to set maximum height. 
• Voxel Radius - smoke voxel size. Smaller numbers create more detail but requite higher particle density. 
• Voxel Resolution - Viewport - voxel resolution size in viewport. Larger numbers are higher quality. 
• Voxel Resolution - Render - voxel resolution size in render. Larger numbers are higher quality. 
• Height - 2D smoke maximum height
• Width - 2D smoke maximum width
• MATERIALS
• Flame elements are separated into 3 elements. Colour ramps for controlling colour, as well as emission strength, are available for each of the three elements here. 
• Spark colour and emission strength available in next panel
• Volumetric smoke colour ramp and density controls available in next panel
• Burn patch colours, bump strength, emission colour, and emission strength available in final panel.