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Practise and exercises.
You have to stretch your entrance.
Use different size anal plugs.

have you tried putting a whole hand in first
using huge dildos first will also help
but yea practice is the key

find appropriate sized plugs/toys that will be representative of the size you wish to achieve. long term wear daily. you need to train it to loosen up. any time its not played with frequently will be causing it to tighten back up slowly. Short play sessions are not going to make noticeable difference. you need that duration to help it increase and stretch more. Also with regards to getting all your fingers in try putting them in with your fingertips all together making your hand into a cone type shape. once in spread your fingers out and then relax. repeat over an over to help get more gape. inflatable plugs are also a good way to work on enlarging the hole.

The key as mentioned by subzzzero is frequency rather than size. Stretching both vaginal and anal is achieved best by long periods with moderate size plugs or dildos, preferably daily. This allows the tissues to relax and conform to the size without risk of tearing. Buy some graduated size butt plugs from Amazon and start out wearing the smallest one as much as possible, as you get used to it move up to the next size and so on. I know plenty of subs that wear a butt plug to work, now I know that isn't always possible but even if you aim for always when you are home you will quickly notice the effect.