Magento 2 offers you the ability to install extensions via their Web Setup Wizard tool. This is the main method used to install extensions purchased through the Magento Marketplace website.
Before completing this installation guide, please read this article to confirm that all minimum server requirements are met. Then follow the steps below:
1. Log into your Magento Marketplace account. Click the My Access Keys link on the Developer Portal tab.
2. Copy both Public Key and Private Key. Note that these are provided on the Magento 2 tab.
3. Log in to the Magento backend panel and go to SYSTEM ⟶ Tools ⟶ Web Setup Wizard.
4. Click on System Configuration. Then, put in the Public Key and Private Key from your Marketplace account.
5. Ensure that your Magento backend is synchronized with Magento Marketplace. To do so, click on the Refresh button. Note that it might take a few minutes to process your request. Once your Magento backend is synchronized with Magento Marketplace you will see your Magento extension overview.
6. Click the Review and Install button below the Extensions Ready to Install text. In the Extensions Grid, mark the checkbox of the extension(s) which you want to install. Be sure to always use the latest version of the extension(s). Click the Install button.
The installation process has started.
7. Readiness Check. The store environment is checked for compatibility. When the Readiness Check completes successfully, tap Next button. If you observe any problems here, fix them and click the Try Again button to retry.
8. Create Backup. By default, the backup includes the code, media, and database but you can deselect any which you do not want to include in the backup. Click the Create Backup button.
Once this operation has finished, click the Next button.
9. This page shows the names and the versions of the extensions which are about to be installed. Click the Install button.
To protect your webstore and customers' data, your website will be taken offline during installation.
10. The the console log, as seen below, shows the current status, and offers you a visual representation of the installation progress.
The Success page appears when the process is complete.
11. Log out, and then log back in for all the changes to take effect. Your extensions are now ready to use.
How to test if the installation was successful?
Step 1: Enable basic settings
Start by enabling the basic features. This can be done by navigating to your store's /admin: Stores -> Configuration -> Cart2Quote as per my guidance below:
Stores → Configuration → Cart2Quote ↓
→ License Information ↓
→ Cart2Quote License ↓
→ Activate Frontend Features: Yes.
→ Customer-Side Options (Frontend) ↓
→ Global Frontend Options ↓
→ Add Button on Category Page: Yes.
→ Dynamic add to cart/quote buttons: Yes.
→ Quotation Cart Options ↓
→ Display Mini Quote Icon: Yes.
→ Enable Mini Quote: Yes.
→ Product Page Options ↓
→ Add Button on Detail Page: Yes.
→ Advanced Options:
Step 2: Test your front-end
Start by going to one of your category pages. Because we enabled "Product Quotable by Default" & "Add Button to Category page" all your products should have an "Add to Quote" button. 
After navigating to the product page you'll see that the "Add to Quote" button has also been added. This is because we have enabled the "Add Button to Product page" setting.
Now let's add the product to the quote and submit a quote request.
Step 3: test your backend
Now that you've made your first quote request let's check it out in the back-end. Start off by navigating to quotation overview: Sales -> Quotations
From here we can navigate to the quote we just created.
On this page, you can fully customize your quote request. For testing purposes, however, let's just submit our quote request. After the quote has been submitted your customer will receive a proposal in his or her mailbox.
See below for an example of customer's proposal email.
If your customer agrees to the proposal and clicks on "Proceed to checkout" the quote will be converted to a regular Magento order. From there your customer may follow the regular checkout process, carrying quoted prices to checkout.
Want to know more about other Cart2Quote features?
Please take a look at our feature comparison list: https://www.cart2quote.com/magento2-quotation-module-editions.html