Now that Axel and Candace have understood the fields of the objects thanks to these articles:
... they are eager to understand the ways to create a subscription and everything that needs to be done afterwards.
These articles explain how to generate a subscription from another object:
Create a subscription ex-nihilo
Candace and Axel know how to set up a subscription from a quotation and an opportunity.
Here are the steps to create a subscription without going through these objects.
Steps
Go to the Subscription tab.
Click on the New button.

Fill in the fields by reading this article: Field dictionary.
The mandatory fields are by default:
Account
Corporate Name
Start Date on the invoicing period
Initial End Date
Frequency

Note: When filling in the dates, pay attention to the calculation explained in this example:
Start date : 01/04/2020
Initial end date : 31/03/2021
Frequency : QuaterlyNumber of occurences: 4
Start date : 01/04/2020
Initial end date : 01/04/2021
Frequency : QuaterlyNumber of occurences: 5
The end date should be in one year minus a day (31/03/2021 instead of 01/04/2021) in order to get the correct number of occurences. Those dates fields are inclusive.
Click on Save.
Add a subscription line item
As with quotation line items or invoice line items, there are two ways to add a subscription line item:
Let's start with the first way, which offers more freedom to fill in information about the subscription line items, such as description, unit price and quantity.
You can also apply a product to this new subscription line items, but it's not mandatory.
Using the New button
Steps
Go to the subscription you just created.
On the Subscription line items related list click on the New button.

Fill in the fields by reading this article: Subscriptions - Subscription line items.
The mandatory fields are by default:
Description
Unit Price excl. VAT
Quantity

Note: As mentioned above, you have the option to link a product to the Subscription line item, but it's not mandatory.
Save.
Now the subscription line item is displayed in its related list:

If you are using products in Salesforce, it is best to go through the Manage Product button to add a Subscription Line Item. This allows you to automatically take the information from the product.
Using the Manage Products button
Steps
Go to the subscription you just created.
On the Subscription line items related list click on the Manage Product button.

In the page Add line items you can see the price book and currency of the subscription.
Beware: If you change the price book or currency, it changes the price book or currency of the subscription and removes Subscription Line Items that do not match the selected price book or currency.
Select the product you want to add as a subscription line item:

Modify the Quantity if necessary.
Note: You can modify others fields value like description / VAT. You can change the fields displayed here by customizing the corresponding field set on the subscription line item object.
Save.
The new subscription line item is also added to the related list:
