a) by abstaining from meat or some other foodIt is so broad, no wonder most Catholics treat Friday as any other day!
b) by abstaining from alcoholic drink, smoking or some form of amusement
c) by making the special effort involved in family prayer, taking part in the Mass, visiting the Blessed Sacrament or praying the Stations of the Cross
d) by fasting from all food for a longer period than usual and perhaps giving
what is saved in this way to the needy at home and abroad
e) by making a special effort to help somebody who is poor, sick, old or lonely, or committing time and effort
to working for justice.
When I was a young priest the lapsed still believed Fridays was a meat free day, because they were lapsed the message hadn't through that they could take options a-e, so for the ordinarily lapsed Friday fish still bound them to the Church, whilst meat eating meant a real distancing from her.