Xanathar's guide contains a section on tool skills that details the content of each tool set, a checklist of skills (for specific purposes) that the mastery of this tool confers a benefit, a possible special use of this tool for those well-mastered and a short list of examples of activities that can be done with the tool, as well as suggested CDs for the necessary capacity checks.
As you have explained, the special use of thieving tools (p.84) is as follows:
Set a trap. Just as you can disable interrupts, you can also define them. In the context of a short break, you can create a trap using the items you have on hand. The total of your check becomes the CD of the attempt to discover or disable the interruption by another person. The trap deals appropriate damage to the materials used to make it (such as a poison or weapon) or damages equal to half of your total check, as deemed appropriate by the Deputy Minister.
As this is quite open, the player and the DM must determine how the trap works.
By and large, the DM is asking for an aptitude check to forge the trap (maybe an Intelligence check with its added skill bonus, due to the mastery of the thieves' tools – or double its bonus of skill, if the character has the expertise with the tools of the thief). This determines the DC to discover and / or disable the trap. The damage caused by the trap is either half of the total control (that is, with a control of 16, it inflicts 8 damage), or another number determined by the DM, with a type of damage determined by the DM according to the type of trap. he is.