Starfish is a wes honey when it comes to getting groomed. To do under her chin, I have her on my knee and gently make a ring round her nose and mouth with my thumb and middle finger, gently lift her head and comb out as much as I can before she scrambles backwards onto the floor.
To do tums and bots I keep their back legs on my knee and lift their front legs and head and hold them firmly in to my tum - which there is a lot of! They are then upright but with back feet on my legs.
Here is the demonstration by my sweetie! I got Gary to take this quickly when I had finished grooming her so I would always remember what her tummy looked like!