Follow installation instructions here: https://telescope.readme.io/v0.20/docs
Add the theme and any other required packages to your /.meteor/packages file. For example, this is what I use:
############ Your Custom Packages ############
welderwire-theme
fourseven:scss
mystor:device-detection
rzymek:fullcalendar
cwohlman:pickadate
mizzao:user-status
fastclick
percolate:intercom
matteodem:easy-search
force-ssl