On the page https://www.tractis.com/ you can see a series of standard contracts.

Through this website you can create, negotiate and sign contracts via online but with legal validity.
The most relevant points in a commercial contract are explained in the Risk Analysis point within the chapter of Consultant Resources.

We emphasize:

• Days of notice on our part and by the agency.
• Payment date from invoice issuance.
• Subsistence or expenses. If they are expenses what is the maximum amount. Detail inside and outside Spain.
• Work during weekends. It must be contemplated if it is a normal rate or with extras. The same for the guards.
• Possibility of being able to work with the final client, directly or indirectly, once we have finished our contractual relationship with the agency that hired us for the first time.