TIPS
This guide mainly outlines which boxes you have to tick to fulfil all application requirements. There are some things you can do to make this ticking a lot easier. Here are few things to keep in mind during the process.
Planning - start early!
Some international universities have application deadlines in December for programs that start in August/September of the next year. Dutch universities usually have much later deadlines, but you do not want to be forced to study somewhere just because you missed every other deadline. Ideally, you already start looking at graduate programs toward the end of your first year.

Year One

After you have settled in at UCR, start thinking about what you might like to study in the long run. When you choose your major, it helps to have looked around at what you can actually do with that major and what courses some graduate programs in your discipline(s) require. If you do not plan ahead in your courses you might find yourself having to take a pre-Master year! If you are not sure what you want to study yet, do not panic – research in your first year is not required, just very much advised.

Year Two
Hopefully you are taking some 200- and 300-level courses in your discipline. This should give you some insight in whether you actually want to spend the remainder of your academic life studying a particular subject (and professionally practicing it, afterward). Many students change their minds at this point. Luckily, it is still early enough to change your program along with it. If you are no longer (or still) unsure what you want to do after UCR, talk to your tutor and/or professors about your options.

Year Three
Consider spending the summer between your second and third year looking at programs and thinking ahead (for example, masterportal.eu is a very good website to start your search). By the end of the summer, you should have selected several programs to apply to and can then spend the next semester working on meeting the requirements. For instance, if a program requires a writing sample, it might be useful to pick a course with a suitable assignment and work very hard on it so that you can use it as a great sample, or edit a previously written paper. The earliest deadlines are in December (especially if you are a non-EU international student who wants to continue in the Netherlands or EU), but even if you can apply later you should have all application material ready in the winter break so that you can avoid stress later on.
Applying
Some of these tips might seem obvious, but be sure to keep them in mind when applying.
Apply to Several Programs
There is always a chance your favorite program rejects you, even if you do everything right. This is not rare - some of the top programs have acceptance rates as low as 10-20% - meaning an overwhelming majority of applicants get rejected. Always apply to multiple programs, both in the top of the field and a little below that. This gives you more options and a "safety net". If you do not want to work or do something other than academics for a year after UCR, but only apply to one or two programs that both reject you, you might find yourself being forced to do something you do not want to do for a whole year.
Obtain some Research Experience
With the possible exception of professional schools, entering a graduate program requires an interest in academics. One of, if not the most important parts of academics is doing research and writing about it. Consequentially, prior experience with these skills is very much valued by academic institutions. Ask professors if they know of any research possibilities. Get the most out of your Senior Project and Academic Internship. Not only does this provide you with experience, you will also have a great writing sample and your supervisor will likely be able to write you a great letter of recommendation. Try to get even more experience in other ways (taking research seminars, for instance). And finally, do not forget to mention your experience in your motivation letter!
Extracurricular Activities
Another important aspect of being a student to mention in your motivation letter is your extracurricular experience. You can show that you are an active student by illustrating that you have been in a board or regularly participated in committee events (e.g. Roosevelt Debating Society). Also, be sure to mention any academic interest that you have outside of your UCR career: maybe you have taken a MOOC or you like to read books about particular academic topics.
Are you sure?
Just because most UCR students go on to do pursue a Master's after obtaining their Bachelor's doesn't mean you have to do so as well. Do not invest a lot of time and money into a Master's if you are not sure you actually want to do it – there's no shame in taking a year or two off to explore other options. Academics are only one of the many routes to happiness, and you should not feel pressured to take a route that does not suit you.