This one is better explained:
1) Open cydia and add this repo: "http://coolstar.org/publicrepo"
2) Tap on search and install this packages: "BigBoss Recommended Tools" "Perl" "iOS ToolChains" "MTerminal" "iFile"
3) Open MTerminal and type "su" than your password (default is alpine")
4) Now type in "installtheos3" and hit return (It will take a while, it depends also by your connection so don't worry)
5) Once in done close MTerminal and open iFile. Navigate to /var and you will see a new folder called "theos"
6) Now navigate to /usr/local/bin and search for a file called "perl5.22.0"
7) Tap on edit, select "perl5.22.0" and copy/link that file
8) Now go to /usr/bin tap edit and than tap on Create Link (Don't paste it!!!!)
9) Serach for the file that you have linked and raname it to "perl"
10) Now we need to fix theos for 64-bit devices so go to "/var/theos/bin"
11) Open the file "bootstrap.sh" and look for this line " if [[ "$(uname -s)" == "Darwin" && "$(uname -p)" != "arm" ]] " and add 64 where you see "arm"
12) The result is this " if [[ "$(uname -s)" == "Darwin" && "$(uname -p)" != "arm64" ]] "
13) Save the file and go to "/var/theos/makefile/targets"
14) Tap the blue info button for the folder "Darwin-arm" and rename in "Darwin-arm64" and save it
15) Navigate to "/var/theos/makefiles/platform" and rename the single file inside that folder in "Darwin-arm64"
16) Now we need to download the sdks. Open safari and go to this website "https://sdks.website " and download the iOS 9.3.3 sdks.
17) Once has been downloaded tap in "Open in iFile" and copy the zip file and extract the content in a new folder that you will create in the next step
18) Go to /var/theos and create a new folder called "sdks". Open that folder and paste the zip file and extract it
19) Now open MTermin and login again with the su mode
19) Type in cd /usr/bin and hit return
20) Now type this " ldid -s clang" and click return. After that type "ldid -s clang++" and click return
21) Now type cd /var/mobile and click return
22) And finally we can run theos's menu. Type in "/var/theos/bin/nic.pl" and click return
23) And here we go the menu will appear!
Credits:ThatsNico99