It is very easy to add a single Contact or Account to a solicitation campaign. Follow these steps:
In the App Launcher, use the Search box to find Campaigns tab.
In the Campaigns tab, open your chosen Campaign record.
Click the Related List section and scroll to the Campaign Member list.
Click View All to show the list of all current members.
You can add Contacts to the Campaign by clicking the Add Contacts button. This will take you to a pop-up window with a Search bar. Use the search bar to locate the Contact you are looking for. When you find them click the + symbol next to the name and then click Next.
On the next screen you can confirm the status that the campaign member will be added with. The default is Sent, meaning all members of this campaign have been sent the correspondence for this campaign.
If the Contact responds by making a donation, the Campaign Member Status will be automatically updated to Responded when an Opportunity is created and associated to this campaign.