It'll take some investment but I'd start by finding someone else that needs a code review and offer it to them first. From there you mention that you'd appreciate if they scratch your back as well.
You can probably find some open source projects where you can offer code reviews to other.
Not sure if this is helpful, but I asked family for code reviews for my birthday. (Requires that you know coders.) This might be helpful: https://www.reddit.com/r/codereview/
I love this! I wish I had programmers in the family.
I personally practice code review on my own stuff - even just me, I'd go through and pick it apart as if it were another person's code.
In some cases I have colleagues / friends who'd be able to help out too
Just checking if it works. Ship it and check for any security issue