If you're going to use YSMenu, I would recommend installing it in addition to (rather than in place of) TTMenu since YSMenu doesn't always run homebrew or download play the same as TTMenu. The two menus can exist just fine on the same microSD card, and you can switch back and forth between them. Here's how.
First install the latest official TTMenu: [*]Download the latest firmware (currently 1.15).[*]Unzip the firmware archive. It should have a file named TTMENU.DAT and a folder named TTMENU (which will contain a bunch of files). Sometimes the TTMENU.DAT file and TTMENU folder are within another folder named "eng". Drag TTMENU.DAT and the TTMENU folder onto your microSD card.[*]Make a new folder on your microSD card called "games."[*]Put two or three .nds files into the "games" folder.[*]Remove the microSD card from your PC. Make sure you always use the "safely remove hardware" to remove the microSD card.[*]Insert your DSTT into your NDS. Insert the microSD into your DSTT. You should get to a menu with your games in it. Click on a game to run it.To install YSMenu for DSTT:[*]Make sure you have the newest version of YSMenu (currently YSMenu0810211800).[*]Unzip the YSMenu archive.[*]Create a folder named "YSMenu" on your microSD drive.[*]Place a copy of "TTMENU.DAT" from the official TTMenu menu into the same folder as the "ttextract.exe" program on your PC. Run "ttextract.exe" on your PC. Place the two resulting files ("ttdldi.dat" and "ttpatch.dat") into the "YSMenu" folder on your microSD card.[*](a) If you want YSMenu to be your default menu, rename "TTMENU.DAT" that is on your microSD to "TTMenu.nds", then place a copy of "YSMenu.nds" on the root (top level) of your microSD card. Make a copy of "YSMenu.nds" named "TTMENU.DAT" on the root of your card. You should now have "TTMenu.nds", "YSMenu.nds", and "TTMENU.DAT" (a copy of "YSMenu.nds") on the root of your microSD card.
(b) If you want TTMenu to be the default menu, place a copy of "YSMenu.nds" onto the root (top level) of your microSD card. Then make a copy of "TTMENU.DAT" named "TTMenu.nds" on the root of your card. You should now have "TTMENU.DAT", "YSMenu.nds", and "TTMenu.nds" (a copy of "TTMENU.DAT") on the root of your microSD card.[*]To customize the YSMenu skin, place skin files "YSMenu1.bmp" and "YSMenu2.bmp" into the "YSMenu" folder. These files are usually not included in the YSMenu release, and are optional. You can download one here if you want (or just skip this step to use the built-in skin that comes with YSMenu):
http://gbatemp.net/index.php?download=3082.[*]Edit "YSMenu.ini". Place a semi-colon in front of the line that says "LITERALS=LITERALS_JPN". Remove the semi-colon from the line that says "LITERALS=LITERALS_ENG". Save your changes to "YSMenu.ini" and place it into the "YSMenu" folder on your microSD card.That should be it. If you are using TTMenu as your default, simply run "YSMenu.nds" to get to YSMenu. Otherwise, it should launch directly into YSMenu and you can run "TTMenu.nds" to get back to TTMenu.
Note that if you don't want a separate "YSMenu" folder, you can place all of the files in the "TTMENU" folder instead, but to make it easier to install updates to one or the other menu, I recommend a separate "YSMenu" folder.
-Bri