The GTAP Data Base provides a consistent snapshot of the global economy. It consists of a set of product-by-industry input output tables that represent the structures of more than 100 economies and are linked by bilateral merchandise and cross-border services trade. It is used to support a variety of models that are built to analyse policy changes. The database is assembled using regional input-output tables and data on trade, energy, protection and macroeconomic aggregates from a variety of international sources. The incompatibilities inherent in the data mean that many compromises are required to produce a fully consistent representation of the global economy. This paper provides a summary of the GTAP Data Base construction process. The process has improved continuously over the different versions. This paper refers to the process used to produce version 8.1. The detailed documentation is available from https://www.gtap.agecon.purdue.edu/databases/v8/v8_doco.asp. The purpose of this paper is to provide a link between that detailed documentation and the higher-level summary in the "Introduction to the Global Trade Analysis Project and the GTAP Data Base" paper by Walmsley, Aguiar and Narayanan (2012). This paper contains three sections. Section 1 provides an overview of the GTAP Data Base construction procedure. Section 2 details the data sources used and the manipulations applied to data obtained from international organizations to ensure that they are globally consistent. Section 3 details the FIT process, which is the procedure used to adjust regional I-O tables so that they are consistent with international data sources.