This can be tough, the best way to do this is on his side.
When you put him on his side you should be able to feel the bladder in the lower abdomen between the hind legs. If you can not feel it easily it may not be that full try waiting about and hour and try again.
You can try using Benadryl this often has a mild sedative effect. This can be dosed at 1-2 mgs per pound of body weight.
Rescue remedy which is an all natural calming agent that is tasteless and can be added to the water so he gets a small amount every time he drinks or dropped on the tongue 10-15 minutes before expressing.
This is going to be very difficult to do on your own, if you can get someone to help you that can hold him on his side by the scruff this may be better. Scuffing with one hand and holding the hind feet with the other is easiest but this does require someone to help you.
If you can get your vet to give you Valium that would be the best route, you may be able to simply give them a call and they can call in a script for you.
Another option would be to see if any of your vets technician staff lives close to you, we often go to clients homes to help and they may be able to work with you to find the best position for your kitty and help you learn how to locate the bladder more easily.
I am sorry this is so frustrating, it is not an easy thing to do which is why unfortunately a lot of these kitty are euthanized. You are doing th best you can for now it will take some time for both of you to get used to doing this