I think a few things you can do is to join in the meetings with the business rep to understand the feature better. I find breaking the code is part of testing. Have a test plan: what's in scope or out. Define how you will test it. Will it UI? API? which test level: system, integration, unit? To me, the most important part is to test the happy path ie functionality. There's BVA, EP and pair wise testing, domain. There are numerous types of non functional testing to perform: check for styles, security, performance, load, cross browser, device. of course, check what's in scope. you can also prioritise: smoke testing, new feature, enhancement, bug fix, functionality, cross browser, OS, regression, non-functionality in that order.
There are numerous testing tasks you can perform: have a register for defects and template for defect recording. Ensure you have a test management tool to record test steps, results, date e.t.c.
Ensure you have a release notes so that you know what's in the code. analyse and propose test types and test levels. have a exit criteria: all high and critical defects are verified fixed e.t.c. have an entry criteria: e.g. new features pass smoke test or app pass smoke test. define and review with team members on defect severity.
in short, have a process and stick to it.