You didn't mention what flash cart you're using, so these instructions are for the DSTT. Other flash carts will be installed similarly.
To install YSMenu for DSTT:[*]Make sure you have the newest version of YSMenu and the latest version of the official TTMenu firmware.[*]Unzip the YSMenu archive.[*]Create a folder named "YSMenu" on your microSD drive.[*]Place a copy of "TTMENU.DAT" from the original 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.[*]Place the skin files "YSMenu1.bmp" and "YSMenu2.bmp" into the "YSMenu" folder.[*]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