Forecasting the individual components

The seasonal, trend and cyclical components can be separately forecast into the future. Putting them together provides a forecast for the time series values,

Forecast  =   Seasonal effect   +   Trend forecast  +   Cyclical forecast

Seasonal effect
We forecast that the same seasonal effects will hold into the future.
Trend forecast
A linear or quadratic model for the trend gives forecasts of the future trend.
Cyclical forecast
The cyclical component can be forecast into the future with an AR(1) model.