Enter pip in the Dos command window and encounter:
'pip' is not an internal or external command, nor is it an executable program or batch file.
This is the situation in the following figure:
The solution-add environment variables
The steps are as follows:
1. Find the location of python installation and find the Scripts folder. Hold down the Shift key, right-click in the Scripts folder, and select "Copy as Path". 
2. Right-click on My Computer and select Properties -> Select Advanced System Settings.
3. Click Environment Variables.
4. In the system variables below, select Path.
5. Click New.
6. Paste the path just copied into the box.
Finally, OK -> OK, reopen cmd.
pip is ready to use.