Open your preferred text editor to create the .ps1 script file.

Use a foreach loop to iterate through each content of SPWebApplication on the local SharePoint farm using the Get-SPWebApplication Cmdlet:

foreach($webApp in (Get-SPWebApplication))

Use a foreach loop to iterate through each SPSite in the Sites property of the SPWebApplication object:

foreach($site in $webApp.Sites)

Verify the CompatibilityLevel property of SPSite to ensure it is in SharePoint 2013 (Version 15) mode and not in SharePoint 2010 (Version 14) mode.

if ($site.CompatibilityLevel –eq 15)

Use a foreach loop to iterate through each SPWeb in the AllWebs property of the SPSite object:

foreach ($web in $site.AllWebs)

Check if the SPWeb object exists:

if ($web.Exists)

Set the master page and logo properties for the SPWeb object:

$web.SiteLogoUrl = "/SiteAssets/logo.png"

$web.SiteLogoDescription = "My PowerShell Site"

$web.MasterUrl = "/_catalogs/masterpages/seattle.master"

$web.CustomMasterUrl = "/_catalogs/masterpages/seattle.master"

Use the Update method on the SPWeb object to save the changes:

$web.Update()

Use the Dispose method to discard the SPWeb object:

$web.Dispose()

Use the Dispose method to discard the SPSite object:

$site.Dispose()

Save the file as a PS1 file, for example, applymasterpageandlogo.ps1.

Execute the script in the PowerShell session:

./applymasterpageandlogo.ps1

Use a foreach loop to iterate through each content SPWebApplication on the local SharePoint farm:

foreach (var webApp in SPWebService.ContentService.WebApplications)

Use a foreach loop to iterate through each SPSite in the Sites property of the SPWebApplication object:

foreach (var site in webApp.Sites)

Verify the CompatibilityLevel property of SPSite to ensure it is in SharePoint 2013 (Version 15) mode and not in SharePoint 2010 (Version 14) mode:

if (site.CompatibilityLevel == 15)

Use a foreach loop for iterating through each SPWeb in the AllWebs property of the SPSite object:

foreach (var web in site.AllWebs)

Check if the SPWeb exists:

if (web.Exists)

Set the master page and logo properties on the SPWeb object:

web.SiteLogoUrl = "/SiteAssets/logo.png";

web.SiteLogoDescription = "My Code Site";

web.MasterUrl = "/_catalogs/masterpages/seattle.master";
web.CustomMasterUrl = 
  "/_catalogs/masterpages/seattle.master";

Use the Update method on the SPWeb object to save the changes:

web.Update();

Use the Dispose method to discard the SPSite and SPWeb objects:

web.Dispose();
site.Dispose();