You really just need one or the other and your straws sound fine. Say I am making a three tiered round cake, I have my first tier down (the larger the tier the more straws you'll need) I would place my straws in with space in between each one-since your straws are slanted stick the flat side down into the cake and cut from there, you can mark or pinch them then pull up slightly and cut with scissors (or since this is pretty new to you, you can use a dowel-stick it in your cake and mark it, gently pull it out and use this as a guide to cut your straws) I've seen it done both ways, just don't stick any I the middle, next I add my next tier and repeat, then add my top tier. Now I like to add a wooden dowel in the center for extra support with larger cakes, take a pencil sharpener to one end forming a sharp point, clean off the debris, and stick it straight down the center. I use a hammer and gently tap it down into my base board. Something to think about is how your transporting your cake, you can add your straws to each tier, place them in individual boxes. I use non skid shelf liner in the trunk of my suv so my boxes won't slide and assemble on site-it depends on what your comfortable with and your design. Also, I always take an 'emergency kit' with buttercream, spatula, my hammer, something to cut my dowel rod with, extra decorations if something breaks, etc. Most importantly each cake should be on its own board:cardboard circle, foam core, etc. and before you put the cake on them they need to be covered in some food safe paper, otherwise they could buckle if they absorb too much moisture from your cakes. If you have any more questions feel free to email me. I hope all goes well!