a couple of options

For a basic connection $session = New-PSSession -computername $serverName -credential $testCred

For a more secure connection $session = New-PSSession -computername $serverName -credential $testCred -Authentication CredSSP

once created you can use Enter-PSSession $session

Remove-PSSession closes it

Things to check

Computer Configuration -> Administrative Templates -> System -> Credentials Delegation -> Allow Delegating Fresh Credentials