Best sequence:
- Human Performance first (it's easy and gives you confidence in passing an exam!)
- Air Law and Op Procs
- RT theory
Do those 3 before you go solo. Next, before your first solo cross-country, get the following 3 under your belt:
- Navigation
- Flight Perf & Planning
- Meteorology
Then, before your Q X-C, pass the RT Practical exam. Finally, during your GH revision before the PPL Skill Test, pass the Aircraft(Technical) exam so that the knowledge will still be in your brain for the Skill Test Examiner's Oral Questions.