Lots of lube, you go very slightly upwards towards the backbone, if anything. Go slowly and gently, and if you get resistance, rotate the tube back and forth whilst pushing slightly, rather than just pushing. Once the tube is in place, you just push the plunger to put the sponge in - that bit's easy.
Wash the tube in warm soapy water between each one.
It wasn't difficult, I did it for the first time last year. For women, it seems sorta familiar, if you get my drift.
There are YouTube videos too, which might give you a bit of confidence.