The r sound in Portuguese is difficult for everybody and whatever is his native language, because there are two different ways to pronounce it and apart from some exceptions like Persian, any language has just one. Guttural r doesn't exist in English. (The few people who speak german has an advantage here. The r guttural Portuguese is very similar to the german guttural r, although in Portuguese this r is sometimes only insinuated. The French r is a guttural r as well.) The guttural r is produced when the stream of air makes the uvula vibrate. In other words, it's the same sound as snoring. To produce this sound simply breathing is not enough. You have to reduce with the back of your tongue the airway. This author would say that most people will find it easy to produce this sound, although it doesn't exist in English. For didactical purposes the guttural r is pronounced very clearly in the following examples and is correct. However in real live the vibration can be very reduced and nothing more is left than something like the german /ch/ in words like Sache (thing) or the Spanish /j/ in words like bajar or even only a /h/ like in health.
The r is pronounce gutturale in the following positions.
the gutturale r
at the beginning of the word
rato (rat), rio (river), rua (street)
when it is written rr
terra (earth), correto (correct), carro (car)
after n / l / s
honra (honour), genro (son in law), enriquecer (enrich), guelra (gil), Israel