inspect your portafilter to group seal annually and if it has gone hard and has cracked as shown in the image above then it is time to replace it
the top, outer edge has a 45 degree chamfer so take care that this edge goes in uppermost as it will make it easier to fit it into the groove in the group
personally i put the thinnest smear of LONDINIUM lube on the chamfered edge to assist getting it into the groove
use the portaflter handle without a basket in it to drive the seal up into the groove – you will need to make more than one attempt
once the seal has been driven a good way in you can place a basket back into the portafilter handle and use the added height on the portafilter to drive the seal fully home
check that you have driven the seal fully home – the bottom of the seal should be flush with the bottom of the group and not protruding downward at all or the exposed section of the seal will go hard in no time at all