When viewing an individual record, you can see a set of buttons across the top of the page, including a Link button:

This gives you a quick way to link that record to another record in the system. The options available will be a reflection of the tabs across the bottom of the screen:

Selecting one of the options - in this example Companies - pops open a search screen:

Clicking one of the entries will link that to the record you are on:

 As you can see, that Company has now been linked to the Activity record:

You can repeat this, linking as many records as required.