1- Open Affinity Designer.
2- Insert your design into a new document.
3- You can select your design as a group and adjust the opacity in the colour picker tool.
4- You can also affect a single object in the layer tab and then adjust the opacity.
5- If you want to apply a transparency independent of the colour you can select the transparency tool.
6- Once the transparency tool is selected, select the object you would like to apply the transparency too from the layers tab.
7- At the top toolbar there will be a type option with a drop down box. this gives you the different transparency types you can select.
8- Select the one that best suits your needs. When you select your option a transparency will appear with several handles on it.
9- There are several points (circles) on the Transparency handles. The black circle is where the most opacity is while the white circles are the most transparency.
10- If the transparency is not the way you need it, you can reverse it by selecting the reverse option.
11- Once you have the way you would like your transparency , you can move the handles around your image and make them longer or shorter to adjust the effect of the transparency.
12- If you select one of the handles circles, you can then go to the color picker tab and adjust the amount of opacity at that point.
13- You can also click once on the handle line and add more transparency circles to add more levels of transparency to your object.