At the bottom of this page there is a checklist of items needed to make a Bill costume.

 Let's start with the shirt.  In Excellent Adventure, Bill wears a sweatshirt, but it doesn't look very much like one!  You need to make a few alterations to get the right look.  First, find an old gray sweatshirt . . . since you're going to cut it up you might want to shop in thrift stores such as Salvation Army and Goodwill (in fact, most of the clothes are best found at these types of stores . . . you won't want to spend a lot of money for the right look!).  The shirt should fit somewhat loose on you, but not too much!  The neck should be high around the neck, but not turtle neck.  No V-necks or low collars.  You'll likely need to get a long-sleeved shirt but that's okay!

When you've found the right shirt, cut the sleeves off so they only go down to above the elbow.  Likewise cut the bottom of the shirt so that it hangs just above your belly button.  Your navel should show when you wear the shirt!  Don't worry of you don't cut it exactly straight, it doesn't have to look neat!  In fact it should look a bit ragged.

Now, turn the sweatshirt inside out.  Yes, inside out, so the fuzzy inside is on the outside!  Now cut or tear a small hole in the front which will be just below your left nipple (when worn).  See photo below for how this should look.
 

Now comes the fun part.  Get some felt tip pens . . . red and black should be fine.  You can use ball point as well, but it will be a lot more work.  On the seam below the left shoulder (when worn) use the red felt pen and just trace that seam from the collar to the armpit.

Next you'll want to write B. PRESTON on the back of the shirt on the collar (see photo below for how this should look).  You'll also want to draw the Wyld Stallyn's logo on the back right shoulder (when worn).  Again see the photo below to see how this should look.  There's also a photograph of the Wyld Stallyn so you can use that as a guide when drawing it.

Let's move on to the shirt Bill wears over the top of his altered sweatshirt (although most of the time he wears it around his waist, the sleeves tied in a knot in front).

This shirt should be a light, long sleeved shirt, somewhat loose fitting.  A cotton or polyester fabric is best, although flannel will work as well.  It should either be blue or purple (as you can see from the pictures below it looks different colors in different light, but it is in fact a light purple).  Because it's practically impossible to find the same fabric and pattern used in the movie, you will probably have to settle for something that looks enough like it.  The exact pattern and color don't matter, the style is more important.  It should be somewhat loose fitting on you but not long (the sleeves can be rolled up, however) and it should button up the front.  When looking for these kind of clothes, thrift stores can be terrific!

Let's move on to the pants now.  Here's where you will definitely want to check out thrift stores, as you're going to be pretty rough with these!  Bill wears worn out blue jeans.  If you have an old pair of jeans around that you don't care much about, that will work great.  Otherwise, check the second hand shops and find a beat up old pair.  They should be straight legged (no flairs, folks!)  They should also wear loose on you . . . the waist should sag down and not fit tightly and the legs should be slightly long on you as you will want to roll them up!  The color shouldn't be terrifically faded but they should be light blue, preferably from wear and wash.

When you've found just the right pair, get out your scissors.  Cut a hole that will be right over your left knee (when worn).  It works best if you start the hole with scissors but then rip it if you can.  See the photo below for how this should look.

Did you keep the felt pens handy?  Good!  Imagine you're Bill sitting in history class and feeling bored.  To pass the time you doodle on anything available . . . including your pant's leg!  On the left leg (when worn) draw an upside down question mark in black felt pen.  Then fill it in with the red pen.  It should be above the hole you've ripped (or if you prefer to draw the mark first and then tear the pants that's fine).  See the photo below for how this should look.

Now this is something I can't confirm . . . I can't even seem to find a good photo to bear this out, but I'm fairly positive Bill also has a tic tac toe drawn lightly on his right left pant's leg above the knee, as if he were bored and playing during class.  Whether or not you want to draw a tic tac toe on the right leg is up to you.

Now on to the boxer shorts.  The waist of Bill's jeans ride so low his boxer shorts always show above them, so it's important to wear boxers (no briefs! ; )  You should try to find a patterned pair, preferably with blue dots or squares or similar.  Now in this case it's probably best NOT to buy these in a second hand shop!  You should be able to find a pair pretty reasonably in a discount store.

On to the feet!  Bill's socks don't really show, so you can wear simple white tube socks.  For shoes, find a pair of slightly worn white sneakers and the finishing touch . . . red shoelaces!  See the below photo for reference to how these should look.

Update: according to ChucksConnection.com, Bill and Ted both wear Converse All Star "Chuck Taylor" canvas basketball shoes.  For more information on "Chucks" than you could ever hope to want, click here.

Finally, Bill wears what are often called Friendship Bracelets on his right wrist.  You can be somewhat creative here and use leather or plastic ones, but there should be several rings either way.  Check the photo below to get an idea of what to look for.

Now this brings us to one of the things that makes Bill so . . . Bill, and that's his hair.  This can be tricky if you aren't naturally blessed with short curly blonde locks!  I personally have blonde hair but it's long and straight.  What I did was go and buy a wig which was about as close as I could come to Bill's hair.  Unfortunately most wigs that you can find that don't cost an arm and a leg are made for women, of course, and while it wasn't bad it did tend to make me look more like Harpo Marx than Bill (and no, no photos, I'm too embarrassed!).  So whether or not you want to seek out such a wig or just make do with your own hair is up to you.  If you do want to get a wig, check local wig shops or surf online to find wig distributors for different styles.  If you go to a shop you might want to take a picture of Bill with you so you can show them what look you're trying to achieve.

Checklist of items you'll need to make Bill's costume:

Light gray sweatshirt (worn)
Long sleeved, patterned light fabric shirt
Pale blue jeans (worn)
Patterned boxer shorts
White sneakers (worn, if possible)
Red Shoelaces
Friendship bracelets (several)
Blonde curly wig (optional!)
Red and black felt pens
Scissors

Have fun!!!!