Where to put header.tex to change page to Landscape Rmarkdown

Question

I read here how to add what is needed to give me the ability to change my page to landscape: enter link description here

However I have saved the header.tex file everyplace I could think of and I still end up getting the following error:

Called from: (function (e) 
{
if (inherits(e, "shiny.silent.error")) 
    return()
handle <- getOption("shiny.error")
if (is.function(handle)) 
    handle()
})(list(message = "pandoc document conversion failed with error 43", 
call = NULL))
Browse[1]> 

I know this is do to me typing \blandscape and \elandscape because the PDF works as soon as I delete those commands. What I need help with is figuring out where to save the header.tex file to so that Rmarkdown knows how to locate it. Thanks in advance for everyone's your help!

Follow up: Taking scoa's advice about adding it to my header I edited the top of my Rmd file to the following. Unfortunately it still didn't work.

---
title: "Lake Report"
output: pdf_document
header-includes:
- \usepackage{lscape}
- \newcommand{\blandscape}{\begin{landscape}}
- \newcommand{\elandscape}{\end{landscape}}
---
\newpage
\blandscape
```{r Non-Seasonal MK results, echo=FALSE}
# MKanalysis <- reactive(HM_MK(data = NonSeasonalData(),n = 4))
kable(MKanalysis(), caption = "Non-Seasonal Trend Analysis")
```
The results of the Mann Kendall analysis....
\elandscape

Latex program/verision: pdfTex, Version 3.14159265-2.6-1.40.17

Answer 1

Save the header.tex in the same directory your Rmd file is. Also, rather than an external file, you could just add those commands in your yaml front matter, a more portable solution:

---
title: "Lake Report"
output: pdf_document
header-includes:
- \usepackage{lscape}
- \newcommand{\blandscape}{\begin{landscape}}
- \newcommand{\elandscape}{\end{landscape}}
---