If it’s really complex I start making a process for it, start to finish, if it’s moderately complex (rebuilding a car for example) I just write out a checklist of everything that needs to be done to finish it and cross it out as I go.
If you mean actually designing the thing and making sure all the parts would actually fit together? I just draw it up in Solidworks. Easier said than done if you’ve never touched CAD software before but plenty of incredible things ngs are also made without this kind of planning, sometimes it’s best just to plow forward and solve one problem at a time.