Here is a summary of the steps to build and install an existing Eclipse plug-in project yourself. We are keeping in mind that there should be a project, but the plugin you are looking for does not hit in the search on the marketplace, or you want to modify an existing plugin and use it.

environment

Windows8.1
Jdk11 --Eclipse version used for build: 2019-03 --Eclipse version to install the plug-in: 2018-12 --Plugin to build: DBViewer Plugin

Installing plugins for plugin development

Install Eclipse PDE. If you have already installed it, such as when using Pleiades All in One Eclipse for Java, skip this step.

Download an existing Eclipse plug-in project

Download or pull any project and extract it to any directory. This time, I am using DB Viewer Plugin on GitHub. I downloaded the zip file and unzipped it to "C: \ app \ project \ dbviewer-master".

Importing an existing Eclipse plug-in project

Import the project you got in the previous step into Eclipse.

Select File> Import> General> Existing Project into Workspace and click Next>.
In "Select root directory:", specify the directory where the project was expanded in the previous procedure, and click "Finish".

You will be able to check the imported project in "Project Explorer". You can check the operation of the plug-in by executing "Testing> Launch an Eclipse application" in "Overview" of "plugin.xml" directly under the project.

Build the project

From here onward, basically, if you work in the order of "red frame" below the page "Tutorials> Create an Eclipse plug-in" displayed in "Help> Welcome", it is OK.

Create a feature

Create a "feature" in the "Plug-in Development" perspective.

Enter the required information in "File> New> Feature Project" and click "Next>". The default is other than the red frame.
Select the target plug-in and click "Finish".

You will be able to check the "feature" added in "Project Explorer". In "Double-click feature.xml> Information", enter "Feature Description" and "License Agreement", and save.

Create an update site

Create an "update site" in the "Plug-in Development" perspective. Here, it is assumed that the shared directory etc. on the file server is "update site".

In "File> New> Project> Plug-in Development> Update Site Project", enter "Project Name:" and "Location:" and click "Finish".
You can check the "update site" added in "Project Explorer". In "Double-click site.xml> Archives", enter "Description:" etc. and save it.
Click "New Category" in "Site Map" of "site.xml". Enter any "ID" and "Name" for the added category and save it.
Continue to select the category you just added in "Site Map" of "site.xml" and click "Add Feature". Select the "feature" created in the "Create feature" procedure on the pop-up, click "Add", and save.

Build & publish to update site

It's finally a build. "Create update site" Select "feature" of "site.xml> Site Map" created in step 4 and click "Build All".

When the build is completed, jars will be created in the "features" directory and "plugins" directory of the "update site" as shown below.

Installation

Start the target Eclipse and install the plug-in.

In "Help> Install New Software", enter the URL of "update site" and click "Add" to display the plug-in you built earlier. Check "Next>"
"Next>" in "Install Detail"
Confirm the license, select "I accept ~" and select "Finish" (This time, we only confirm that the installation can be performed, so the license is not entered.)