Am using the Picsart application for this purpose.
Mobile model: Samsung Galaxy S3 GT-I9300
Android version: 4.3
Picsart version: 4.5.5
In the homescreen of picsart app, select Draw option & then "Draw on background" in the sub menu
Choose the background. As I want finer precision, I use the graph sheet background.
Set measurements. There are standard templates available. U can use that or set measurements according to need. I wanted a 20×14 square cm area for drawing in a A4 size sheet. I did some rough calculations and then fine tuned it to 980×1435
Select the layers option, then "photo layer" in the submenu. Add the required photo. U can also use the "add photo" menu if the grid doesn't permit u to add the wanted field area. But I always prefer photo layer as the placement will be perfect.
Now in the layers menu, Reduce the opacity of your photo. U will be able to see the grid over the photograph.
Use the "save image" option to save your work and you are done and all set to portrait drawing. Not just for portraits, any reference can be made this way. It is only a basic idea & there are many more methods to experiment yourselves.
Create. Have fun.
Tips n tricks: if you want to make this easier, to get a better idea about the lighting and shading, first edit your photos with sketcher/drawing effects using any application. There are many in the market. (picsart provides it too in the Effect section, but I would also recommend sketchguru) When you are satisfied with Fx, save & use that image for applying the grid.
HERE is how I continued with this if you are curious :)