After you establish a contract, you’ll need a {0} and a {1} with modules fit for the contract. For example, if you’re trading {2} for {3} you’ll need to have both a Unit Module ({2}) and a Fluid Module ({3}) attached to the {1} (1). Select either module, click 'assign a contract' (2), and assign the contract you wish to begin (3). Note that any other attached module will be auto assigned to the appropriate type of material if compatible with the contract.