Yes it is hard outside, but if you fix the testicle hard, so it is easy to push it in with a bit slowly pressure... and it will slide in very easy then.
If you not fix the testicle and try to hold it only with the fingers, you will have any pain and/or you are not able to penetrate it.
For injecting, use the normal upside of testis and then go around in the inner of testis to inject the alcohol in multiple areas.
The testis is very smooth in the inner and you can slide around with the needle without pain.