Thread a needle and run it through the blister, snip the thread off and leave it as a drain.
Spot on
Leave the small ones alone. Use thread on anything larger than your thumb print.
Make sure the needle is clean - boil or medi-wipe it
Larger blisters need to be checked because the fluid can track under good skin and make the problem worse
Any open blisters keep clean and dry and covered. Clean them twice a day and use baby powder to keep the rest of your foot dry whilst training