I want to use a silicone cage, what should I pay attention to

This is probably the worst design you can find. Ring part of it is thick and has sharp edges so it will probably hurt and I believe it is not silicone but a very hard and cheap plastic. I had one of those, now it's in the trash.
 
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Get steel. Those silicone ones are terrible. I got one with a large ring and they still felt sharp and uncomfortable. Just get a steel one and get the ring around the cock and balls larger than you think. The cock cage can be smaller, just make sure you can shove your cock into it.
 
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I'm addition to the other comments, that type of cage might be fun for an hour or two, but it is not the sort of thing that you can wear for extended periods (days or more). It is impossible to keep it and yourself clean.

As others have said, go for an open wire type cage instead.