Please follow these steps to access Phi on the Onshape App Store:
Step 1
If you do not have an Onshape account, select the plan that best suits you here. If you are already an Onshape user, move on to step 2.
Why do I need an Onshape account?
Phi is a freeform design tool, that can be found in the Onshape App store. This design type is complementary to Onshapes core, which is mechanical CAD (MCAD). To see how Onshape ranks in relation to other software in the same field, check the G2 grid.
Step 2
Log onto Onshape, and visit the Onshape App Store. You will find Phi in the "Design & Documentation" category:

Click on "Subscribe". You will be asked to re-enter your password, and you may need to accept terms and conditions. You may also need to log out and log back into Onshape for the subscription to take effect.
Step 3
Create a new document or open an existing one:

Step 4
Once you open the document, click on the + sign at the bottom left, and then click on the Phi application:

Step 5
A new Tab will open at the bottom of your document, click on it and start designing in Phi!
