The word however has several meanings.
But it can never be used to join two sentences.
However sometimes means nevertheless.
CORRECT: It is very sunny. However, I won't go swimming.
For some reason, this use of however causes many problems! You either make however mistakes often, or not at all. (You probably don’t realize this.)
How to stop making mistakes with however forever
How common is the however mistake?
I searched with Google for "however it".
If you suspect you make this mistake, follow our rule.
(However, if you're a sophisticated writer and are absolutely, positively certain that you can handle the dreaded H-word correctly, feel free to use the word in other ways and other places, for example as a mid-sentence adjective.)
The Contented Painless Grammar course has dozens of other tips to save your reputation as a professional, grammatically correct writer.