1- Open up Affinity Designer.
2- On the top toolbar select File > New.
3- A new window will appear.
4- When creating a new document there are several options you need to take into consideration.
First is the type of document you would like to create. this can be selected via the Type drop down menu.
5- Next is the Page presets, here you can choose what size document you need.
6- Next is the document unit size. you can choose to set your document in pixels or many other different measurements.
7- If you want to create an artboard you need to select this box. (note that you will be able to create an artboard if you create a document first.)
8- Colour format can be changed, but for most projects the preset RGB/8 will be fine.
9- The dimension option you can change the size of your project to specific pixel and the DPI. this is good to use if your creating web files or other files that may needs a lower DPI.
10- If you select the Include margins box you can add margins to your project. This is useful when creating print documents.
11- when you are happy with your settings click OK. Your document will appear.
12- At this point you can start creating. But if you want to create an artboard and have already started your work you can select the artboard tool.
13- Once the artboard tool is selected you can now select the size of your artboard (1) and then select insert artboard (2).
14- Your document will now appear as an artboard.