I'm designing a crochet blanket for the first time and I need some help.

[Kayla Jackson]

I'm very good at following other people's patterns (in fact, I have won a number of awards at the ND state fair) but this is the first time I've made my own pattern.

Okay, so here are the stats: 
Name: Colorado Larksfoot/Ombre Blanket 
Size: 50 x 60 
Stitch: Larksfoot/Icicle 
Yarn: Lion Brand Heartland (251 yds/skein, according to lion brand website) 
Colors: purple, blue, green, and brown (in that order) in an ombre colorway, having (hopefully) three shades of each color from dark to light. 
Hook: H. I did a test with some leftover heartland from another project, and found that I prefer this hook one or two sizes down from the yarn's suggested hook as I crochet pretty loosely and don't want the blanket to be too holey. 
Note: I also want to put some sort of border on it, not sure what color or border yet. Suggestions would be helpful. 

Now, I'm trying to figure out these things: 
1. Number of skeins in each shade to buy (including border) 
2. Number of rows in each shade 
3. Type and color of the border 

It's my first time designing my own pattern so I'm not sure how to calculate #1 & 2. I'm also not sure what kind of border looks good with this stitch or what color to do it in. Can someone help me, please?

[redrosesdz]

To do 1 & 2, make a gauge swatch with the yarn, hook size and stitch pattern. Make it 5"x5". Measure the middle 4" across to get your stitch count and the middle 4" vertically to get your row count. From this you can figure out how many stitches across and rows you need for the size you want. Next mark (paperclip works) or cut the end of the yarn used for the gauge. Unravel it and measure how much you used for 5"x5". Use this to calculate the number of yards/meters. OR weigh your swatch and use it to calculate the number of ounces/grams.

For borders, I like to make the afghan first. Then I decide if it needs a border, edging, or nothing. If I do a border, it's rarely what I first imagined. I test going around the corner and a couple of inches on the side to see what I like. 

[Granny Square]

I agree with making a swatch and measuring it; you wouldn't have to cut it if you didn't want to, you could put a safety pin though the yarn strand at the end of the square, unravel, and measure from the safety pin to the end.

Here is a pattern for 5 colors, in nearly exactly the same measurement you were aiming for. http://www.yarnspirations.com/patterns/bernat-super-value-larksfoot-blanket.html?id=199372  I think it looks fine without a border, IMO, but up to you.  This uses 6 skeins of the same weight of yarn x 426 yards per skein = 2556 yards.  One of the colors uses 2 skeins, the rest 1 each.  This is just a ballpark figure for yardage you'll need to buy, as your choice has a smaller put-up; you will probably need 2x the number of skeins to get the yardage needed.