The simplest way is to request a lanyard at the support desk. If your mum (or you if she is not keen) wear it you can sit in the assistance area in the terminal if there is a shortage of seats elsewhere, you will usually be allowed to go in the first class queue or taken to the front of the check in queue, directed to fast track in security and given help if needed and taken to the front and given help in passport control and avoiding the electronic booths. You may have to wave the lanyard if it is not noticed but staff are getting better at noticing it. You will also be able to board in the first group if you request it at the gate. In some cases there are sunflowers showing you the route to take through the airport. There are also quiet rooms to sit in but the ones I have looked at are not very pleasant I think they are aimed at those who can't deal with the sensory overload at the airport.
You can request assistance and travel through the airport with those in wheelchairs but then you are dependant on helpers being available - I have seen people getting very anxious when no-one has come to take them to the plane. If your mum is physically fit I would recommend going it alone with just the lanyard.
You may want to prepare for security before going in the queue - my OH is reluctant to give up his watch etc so I find a quiet corner to prepare for security, checking pockets etc.
I hope this is helpful and you and your mum have a lovely holiday. I have recently been to Spain and found everyone very helpful and the waiters in particular were very understanding of my husband who rather rudely shouts for drinks!
