1) install git: 

windows:
http://msysgit.github.io/

linux:
sudo apt-get install git

2) create folder for work 
example: 

mkdir build_v8
cd build_v8

3)
git clone https://chromium.googlesource.com/chromium/tools/depot_tools.git

add PATH to depot_tools
linux/mac: 
export PATH=`pwd`/depot_tools:"$PATH"

4)
gclient

windows:
add to PATH variable ...build_v8/depot_tools/python276_bin;

5)

под виндоус есть баг. может в будущем исправят. пока нужно ДО fetch вызвать
set DEPOT_TOOLS_WIN_TOOLCHAIN=0

fetch v8
cd v8

6) build

linux:
CFLAGS="-fPIC" CXXFLAGS="-fPIC" make native

mac:
CFLAGS="-fPIC" CXXFLAGS="-fPIC -stdlib=libc++" LDFLAGS="-stdlib=libc++"  make native

windows:
generate visual studio project
32bit: python build/gyp_v8
64bit: python build/gyp_v8 -Dtarget_arch=x64

build project in visual studio (не из-под админа, иначе переменные среды тоже для админа надо)
a) open all.sln
b) select projects:
icui18n
icuuc
mksnapshot
v8_base_0
v8_base_1
v8_base_2
v8_base_3
v8_libbase
v8_libplatform
v8_nosnapshot
v8_external_snapshot
с) Project Properies -> Configuration Properties -> C/C++ ->Code Generation
for Debug Configuration change "Runtime Library" to "Multi-threaded Debug DLL (/MDd)"
for Release Configuration change "Runtime Library" to "Multi-threaded DLL(/MD)"

Rebuild All

7) Rebuild doctrenderer project