Professional chef’s fill a piping bag whilst holding it in one hand, as shown here. Another method is to pull the sides of the bag over the rim of a jug to act as a support.
Fit the nozzle inside the bag and twist the bag above it to seal the nozzle.
Make a collar by folding the top of the bag over your hand and spoon in the filling.
When full, clear any pockets of air by twisting the top of the bag until the filling is visible in the nozzle.