First preserve the actual result list. Record the query, date, time, device, region, language, filters, visible products, notices and the directly accessible ranking information. An isolated screenshot without search context can shorten the overall impression.
On the operator and seller side, add agreements, payment models, commission data, product feeds, ranking configuration, test records, approvals and change logs. For automated systems, preserve the configuration used at the relevant time. A later adjustment does not prove what the earlier result list showed.
Separate the alleged defect. Is it a missing disclosure, misleading ranking information, a particular product statement or a contract-performance issue? The article on a UWG cease and desist letter shows why the letter, claim and requested undertaking should be reviewed separately.
If the placement continues, an injunction or interim relief may matter. The result view, risk of repetition and protective objective should be documented separately.