How can i set up and apply multiple masters?

Hi Leia, welcome to the forum, thank you for your post, in answer to your question; In PowerPoint, all aspects of the presentation that you create are based on Master or Global settings. The master settings can be changed at any time during the creation of the presentation. You can also specify that certain aspects do not apply to certain slides by switching them off and at a later stage re-applying them.
A Slide Master is an element of the design template that stores information about the template, including font styles, placeholder sizes and positions, background design and colour schemes, you would typically use the Slide Master to do the following;
1. Change the font or bullets
2. Insert art such as logos - that you wish to appear on multiple slides.
3. Change place holder positions, size and formatting.

You can also create multiple Slide Masters which can be used at different stages in your presentation, perhaps a change of subject or different set of slides to a different audience.
To create a second and subsequent masters follow these steps;
Switch to Master View, Choose Insert>Duplicate Slide Master, you will notice that your new master is now shown in the left pane. Select the first master, click the Preserve button on the toolbar to save the master style.
Select the new master and make any formatting changes as shown per above, and then save and preserve the master and then save the presentation.

You can now apply the new master to selected slides in your presentation; > Switch to Slide Sorter view, select the slides to which you wish apply the new style, in the Slide Design tab note the Used in this Presentation section, select the master style to apply, and click the option; Apply to selected slides, save and view your presentation.
I hope that has helped, if so please click the Resolved link, best regards Pete.