In general there isn’t an “app for editing”, the usual case is start from an existing part and use parts editor (which I don’t!) and an svg editor to make parts. Instructions that apply to current versions of Fritzing are available here:
I lately learned there aren’t links to the videos in Old_Grey’s tutorial so you need to do a google search for the title and then they come up on YouTube.
That said, making parts is difficult and if you only need a small number of parts probably not worth doing. In such a case you should fill in the template in this category with a web site that provides the dimensions and connector information for the part you want (or first do a google search of the form “fritzing part part_description” where part_description is the model number or description of the part. That will find any existing parts (and a search for “fritzing part ttl rs485 module” turns up a variety of hits) and is the first step in making a part (don’t reinvent the wheel!)