You can attach the boat to the anchor with rope, chain, or a combination of the two. Rope is the overwhelming favorite, usually with a length of chain at the anchor end. Use nylon rope only. Nylon rope is strong, light, easy to handle, and elastic, the latter a most desirable characteristic in an anchor rode. Three-strand offers the dual advantages of greater elasticity and lower cost, but where the anchor line will be fed through a deck pipe for stowage, the added flexibility of braided nylon can make it a better choice.
Determining how long your anchor rode should be is as simple as multiplying the deepest water you expect to anchor in by 8. As for rope size, the rule of thumb is 1/8" of rope diameter for every 9' of boat length.
So if you expect to anchor your 26' boat in 30' of water, you need 240' of 3/8" nylon rope. Unlike oversizing the anchor, oversizing the line is not recommended because that reduces its beneficial elasticity. As a practical matter, however, rope with a diameter smaller than 3/8" is difficult to grip.
In an emergency the line can be tied to the chain or the anchor using an anchor bend, but for regular use give the end of the line an eye splice around a thimble, and shackle the line to the chain.