The best time to check yourself is after a warm shower or bath as your skin is most relaxed; so take the time to sit back and relax before you get hands on.
Only examine one at a time by taking the testicle between your thumb and first two fingers and roll with consistent gentle pressure. Feel for any lumps or changes in shape, size or consistency. Familiarise yourself with the epididymis towards the back of the testicle. It should feel rope like and tender to pressure, this is normal.
If you notice a change or you feel a lump, don't panic. Most lumps are not cancer but if you do notice anything unusual or anything concerning you should visit your doctor as soon as you can. If caught early enough testicular cancer is highly curable.
Most men have one testicle that is larger than the other or find that one testicle may hang lower than the other or both of these. This is very typical and is definitely not a reason to worry.