This document, produced as part of the EU Horizon 2020 PlasticsFatE project (Deliverable D2.3, Annex III), provides comprehensive guidance on best practices and method validation for the detection and quantification of microplastics (MPs) and nanoplastics (NPs) in diverse matrices. It emphasizes the critical importance of integrating quality assurance (QA) and quality control (QC) principles. Given the heterogeneity of analytical techniques and sample types, the document outlines tailored strategies for method development and in-house validation, including protocols for evaluating performance parameters such as accuracy, precision, trueness, limit of detection (LOD), limit of quantification (LOQ), selectivity, and robustness. Special attention is given to the validation of both number- and mass-based quantification methods, as well as particle sizing techniques, highlighting the need for flexible and purpose-driven validation frameworks. The guide also addresses error detection via different types of QC samples (e.g., blanks and spikes), and the estimation of measurement uncertainty using top-down and bottom-up approaches. In addition, it covers the validation of software tools used in data analysis and provides detailed recommendations for transparent result reporting. The aim of this document is to improve reproducibility, harmonization, and scientific integrity in MNPs research, supporting regulatory relevance and risk assessment.