After 1-2 minutes of soaking time, take each club in turn and use an old toothbrush (or other plastic-bristled brush) to clean out the grooves on the clubface. This is the most important step in cleaning your clubs - removing all dirt and debris from the grooves.
Also drag the brush across the sole of the iron and over the back of the clubhead, removing dirt, grass and other debris.
A soft-bristled brush should work fine. If you've allowed dirt to build up in the grooves and harden over time, you may need to allow more soaking time and then use a stiff-bristled brush. Never use a wire-bristled brush during cleaning.