It would be very hard to give a "quick" description of sensors and where they should be placed.. it would take several pages of posts to explain this, if your new to the build your best starting point is to read the epson service manual for the particular printer.. the concept is the same for all current desktop epsons...
you also have the clue book avail on colorfingers wiki or within this thread here on page 39 post 583 ish... your best off reading the entire thread/s from beginning to end for all the trials and tribulations of the build/s and the best methods for each with input from several builders whom have completed working builds

best advice i can give..
it not only contains build info but user experience with dtg, you may find its not best for what you need to achieve and would save you lots of time and effort $$, if this in fact the case... you will also have a learning curve with the rip and/ciss systems/white ink printing/pretreat process etc.. i dont want anyone to think this is push a button profit because its certainly not.. in fact we have found screen printing a better profit margin/ process for what we are after.. this of course after traveling the hard road of the diydtg build and thinking its easy street. read as much as you can throughout the forum on the different processes and user opinions...
best of luck...