Either microwave them as they are self sterilising or we used the Milton cold water bucket!
We've always used cold water bucket with Milton tabs and it's worked fine.
Honestly I swear by the cold water bucket! It’s less stress and you haven’t got to get the bottles assembled and done within a certain time frame after sterilising. You can leave them in as long as you like and the water’s sterile for 24hours x
friends recommend this one https://www.philips.co.uk/c-p/SCF291_01/avent-steriliser?utm_id=71700000121536327
I had mam ones with my son and just microwaved them if they are the self sterilising ones. Just washed them and pit in microwave for a round 3 mins and let cool, put back together and done