IKEA is just the brand name, or business name, of a global business; it's a name just like "Apple" or "Google" or "John's Grocery Store".
A person who founds a business can give it any name they want (so long as no-one else legally controls that name). The founder of IKEA was a Swedish businessman called Ingvar Kamprad. He made up the name by using his own initials (i.e. the initial letters of his first and last name) and then adding on the first letter of the farm where he grew up (Elmtaryd) and then the first letter of his home town (Agunnaryd). Hence, the made-up name is "I-K-E-A".
The IKEA brand has developed its own particular styles of furniture and furniture assembly and even instructions. The IKEA-brand instructions generally involve no words but very clear drawings on how to assemble the furniture, step-by-step. As a result, many people will refer to "IKEA-style" to mean - for instance - furniture that is delivered packed in a box (as a "flat-pack"), ready for the buyer to assemble it themselves, or even to the instructions.
The IKEA-style instructions for your guitar would therefore mean instructions that are clearly laid out, step by step, probably with clear drawings but few words.