My input is to first and foremost agree on a
safeword. A safeword or phrase at a minimum is the means to communicate in a domination/submission scenario. For example if part of your girlfriends tie-up kink is to playfully struggle or resist, which is not uncommon, how is she supposed to seriously say "no"? Easy, an agreed upon safeword which one would be unlikely to utter in a sexual situation such as "cat litter".
It is important to agree upon a safeword for many reasons the most important of which is well safety. The submissive, in this case your girlfriend, needs to be able to explore this area of her sexuality with the assurance she can stop any situation which makes her uncomfortable. The dominant, in this case you, is protected from accusations of date rape or abuse (to some extent).
Finally, there are zillions of ways to tie someone up. Do some Internet searches for erotic bondage and you'll find lots of resources.