I'm working on a book right now, and I want to add something that the reader really has to search for, such as symbolism or motifs. What are some ways to implement those, or such like it, into any type of writing?
Oh Dear, no. That is not how it works. You should always make your work as clear to the reader as you possibly can. If your use a symbol or a metaphor, it should be to make your meaning clearer, not more obscure.
Yes, I know that in school your English teacher told you that there were deeper meanings hidden in the text and expressed through metaphors etc. etc. That was bunk. The reason we have to study the metaphors used in the literature of the past is that all stories are told by reference to other stories -- stories the reader knows. When the original author wrote those works, the stories they referred to were familiar to the audience of their day. But those stories are not so familiar to us today, and so we need to study up on those stories in order to understand the reference the author was making to them.
The most frequent and obvious examples of this are biblical allusions. When Shakespeare wrote, his audiences would have been thoroughly familiar with the bible. Most modern audiences are not, and so they need to have the bible stories that Shakespeare refers to explained to them before they can understand his meaning.
It is true that some of the early moderns indulged in highly obscure references that few of their own time would understand. They were rewarded appropriately: very few people read them.
It is also true that some of the things that authors want to say are difficult to express, and that they have to be approached not by simple statements but by the juxtaposition of many ideas and images. We may not all readily understand these ideas because they are inherently difficult to understand, but if the author is any good they have done their very best to make their meaning clear.
So, whatever it is you are trying to say, say is as clearly and as simply as you can. Don't use a metaphor or a symbol unless that metaphor or symbol is the clearest way to accurately express the thing you are trying to say. And if the thing you are trying to say is a simple straightforward thing that you can express in a simple essay, write a simple essay.
You could use symbols and motifs. This sounds very obvious, but like all literary techniques, if done well it works. It could be the use of the colour red. It could be the mention of a poetic work. It could be the constant use of metaphors connected with the sea. Think of the symbols and motifs and then insert them at appropriate places.
The best way to add deep meaning is to make your writing real and relevant. There is no secret element to make your work seem edgy and subtextual. Just make your story brutal and smart. That will give all the meaning you need.