Step 1: Add sp-mode WiFi to your docomo account: (if you don't have it already)
Walk into any docomo shop and ask for the 永年無料 WiFi campaign if they don't already offer it to you. You can also add it yourself at mydocomo.com:
Step 2: Setup your sp-mode WiFi credentials: (if you don't have them already)
Go to the docomo portal "d menu" on your phone's browser at https://smt.docomo.ne.jp/ - it might work with Chrome too, but I recommend using the regular Android browser to be on the safe side.
Click on the お客様サポート and go to spモード settings
Here you can change or retrieve your WiFi username and password for use in Step 3.
Step 3: Bring your laptop to somewhere with docomo WiFi: (I recommend Starbucks!)
Connect to the WiFi network called "docomo" and select WEP Enterprise security:
Enter your sp-mode WiFi username as "jmaurice-spmode" if your username is "jmaurice", with your sp-mode WiFi password from step 2.
Step 5: Accept the docomo X509 certificate on your laptop if prompted: (and "Always Trust" it)
You may have to install a new certificate when using WiFi for the first time at a new location. For example, train stations and Starbucks typically use different certificates.
Step 6: Enjoy docomo sp-mode WiFi on your laptop: (even if it is a bit slow)
Note that you can only use sp-mode WiFi from one device at a time, so your phone has to be on 4G while you use WiFi on your laptop.
You might get disconnected from time to time, depending on how much docomo likes you or not.