So first of all, download and active plugin. You can download it from here:

https://wordpress.org/plugins/nextgen-gallery/

1. After activation, go to management sidebar and choose Add Gallery / Imagesas shown below.

2. After click on Add Gallery / Images, you’ll see below page. This page is shown when you use NextGen Gallery for the first time and you need to choose a name for your gallery here.

3. After choosing and entering your gallery name, click on Add gallery. Now, you’ve access to upload page which let you upload your gallery photos by clicking on Browse. (You can upload multi photos at the same time.

4. When uploading process is done, go to add new page/post section and choose NextGen photo gallery button from editing menu.

5. Finally choose gallery and showing style in new windows.