The first few tips apply to picking patterned paper.
10-12 Patterned Papers is plenty! Don't put too many papers because you will get bored with the kit before you've "killed" it. If you put too few, you don't have enough to work with. If you have a 6 x 6 Paper pad that you want to throw in there, do it.
Sometimes it all comes down to "what do I like better?" If you find yourself having a hard time whittling down your patterned papers to just 10 or 12, you just have to choose what you like. Make sure you have a good balance of colors and pick what you love.
Make sure you have a good balance of prints!
busy vs. restful
small patterns vs. large patterns
multicolored vs. solid
tight patterns vs. loose patterns
papers that could be used for backgrounds vs. papers that steal the show
florals + stripes + dots + text + texture
It's important that your papers have a good mix of patterns that pair well together.
When mixing collections, look for similar tones and styles! You can see from the photo below that all of the papers go together really well. That's because, even though I used several manufacturers and papers that were newer and some that were older, I looked for papers that had a similar tone and style to them.
Try to find something that incorporates several of your kit colors! I have learned that I can usually find a sticker sheet or die cut package that includes multiple colors that will work with my papers. It's okay if every single sticker/die cut doesn't match. That being said...
Don't be afraid to split up your packages! For this kit, I had a package of tags from the Crate Paper Notes and Things collection. About half of them had the right coloring, so I just pulled them out. I'll save the rest for another project.
Try to include a variety of textures and dimensional items! Try to include items that are flat, like stickers and die cuts, as well as more dimensional items like chipboard or flair. Include things that can be tucked or layered, like tags. I always try to include one or two trims, like washi tape, ribbon, or twine. This will keep your layouts from feeling flat and boring.
Some final words...
Include what you've been meaning to get to but haven't! What I mean is that if you have some awesome peerless watercolors that you were so excited to buy, but haven't used at all, throw them in with your kit if you think it will get you to use them. If you bought an amazing package of gold foil die cuts but haven't had occasion to put them on a layout yet, throw them in with your kit. .
Include what you love! It does you absolutely no good to have a bunch of stuff that matches well and that you don't really like. What is enticing you to use that kit? Make sure you include things that you love!
There are no rules! I've put together a kit that has 11 patterned papers, 1 specialty paper, 2 cardstocks, 1 sticker sheet, 1 diecut package, 1 set of rub-ons, 1 sheet of label stickers, 7 flair buttons, and two trims. That doesn't mean that you have to do the same. Of course, you can do whatever works for you. That's the beauty of making your own kit, isn't it?
I made a video describing my whole process of putting together this particular kit, and I'll do it again each month. Have questions? Watch the video. It might answer some of them.