Use some long nosed pliers, grab a hold of the bent part of the clip, place the non-bent part into the groove in the piston and pull back the pliers until the clip will enter the groove properly, harder to explain than it is to so I think. Also best to do one side before fitting the piston.
And it is important to place rags in the crankcase mouth until it is fitted properly just in case you drop one of the clips.