Yes, preparing the declaration of conformity is something you can in principle do yourself. It is not a certificate that an authority issues for you, but a declaration that you as a manufacturer sign yourself, based on data that you collect and record yourself. The question is not so much whether you are allowed to do it, but whether you currently have all the underlying information ready to substantiate that declaration in content.
The declaration of conformity pursuant to Annex VIII is in essence a summary document: it describes the packaging, refers to the applied standards or test methods, and confirms that the essential requirements have been met. That declaration never stands alone. It relies on a technical file with material data, weight, recyclability information and any test results. Whoever has that file in order can derive the declaration from it. Whoever still has to build that file is in fact doing two things at the same time: collecting data and formulating it correctly.
What you can realistically do yourself
The administrative part is something most companies can do well themselves. Think of requesting material specifications from your supplier, recording weight and composition per packaging component, and keeping track of which standard or guideline you have applied. Once that data is collected, completing the declaration itself is a matter of precision: stating name and address correctly, describing the packaging unambiguously, and having the declaration signed by someone authorized to do so within your organization.
Where it becomes more difficult is in substantiation that goes beyond your own administration. Think of verifiable information about heavy metals or PFAS: this typically requires a test report from a laboratory or a declaration from your supplier, not your own assessment. The same applies to recycled content percentages that you state: these must be traceable to a source, not to an assumption. Without those underlying documents, the declaration is written, but not solidly founded.
Where the line usually lies
The line between doing it yourself and outsourcing lies in practice less in the question "am I allowed to do this" and more in the question "do I have the time and resources to do it completely and consistently". Companies with one or two packaging types and a manageable supply chain often manage well independently. Companies that supply in multiple countries, carry multiple packaging variants, or work through multiple suppliers more quickly run into the problem that data becomes scattered, versions no longer match, or it is unclear which declaration belongs to which packaging.
In addition, a declaration of conformity is not a one-off document that you file away in a folder. If your supplier, material or packaging form changes, in principle the substantiation changes too. Anyone who tracks this manually must devise their own system to know when a declaration needs updating. That is precisely the part where guidance often saves time: not because preparing it is complicated, but because maintaining it requires structure.
First check whether it applies to you
Before you spend time on a declaration of conformity, it is useful to first be clear about whether you fall under the regulation as a manufacturer for the packaging in question, and in which country that obligation specifically applies. This varies by packaging type and by role in the chain. see in a few questions which role fits you provides a free initial indication of this, including a reference to the applicable text.
Once you know the obligation applies, you will find in the knowledge base with explanation per subject further information on what exactly belongs in a technical file and a declaration of conformity, and how these two relate to each other.
For companies that prefer not to set up their own system to keep files and declarations up to date, epria.eu builds the template and documentation along with you, per packaging and per country. What this costs and what exactly is included is clearly set out on the page with the subscription options. How the underlying information is developed and what it is based on can be read on the page that explains where the information comes from.
If you are still unsure whether it is really necessary for your packaging and your situation, or if you simply want to first see what has already been arranged for you before you get started yourself: the free role finder provides clarity on this in a few minutes, without you needing an account for it.