Test your site with intellectually disabled users. Even better, involve people with intellectual disabilities when you are building your site.
Use lots of images. Images are important. They are core content, not just decoration, for people with intellectual disabilities. Use icons for links, and symbols to supplement text.
Keep the menu choices simple, and limit how many there are. Five is enough for this user group.
Use big text. Keep your writing simple. Use short sentences, short paragraphs and simple words.
Include audio versions of all content, so users can listen to it instead of reading it. Video and animation also go down well with this user group.