I think that just regular white glue would work fine.  If you could lay a book or other weight on top while it dried, it would make a tighter connection. 

Be sure to let it dry for a few days before you paint it, as any trapped moisture could cause problems.

A tip on putting the letters down:  with many alphabets, the rounded letters (a, c, e, o, s) are made slightly taller than the other letters.  Check the o against an 'i', upper or lower case, and see how much larger the o is, if at all.  Some kinds of letters are all the same height, so you don't have to worry, and you just space them apart the way you think they look best, and put them all the same on your line.

Lay a ruler down where you want the letters to go, and make a light line for the baseline.  Line up the letters like you want them to go, but don't glue them down yet. 

If you're using all capital letters, they are easy, and only Rule #3 below applies if O or S is larger than I, L or T (etc).

When you've got the placement right, lift up one letter at a time to glue it down, and go by these rules for upper and lower case letters:

1.  Place any letter with a flat bottom (the 'foot') so the foot is right on the line (f, h, i, k, l, m, n, r, t, v, w, x, z).

2.  Place any letter that has both a 'foot' and a rounded bottom (b, d, u) so the 'foot' is right on the line, and the rounded bit usually goes just a tiny bit below the line.  Keep the vertical parts straight up and down.

3.  Place the rounded letters (a, c, e, o, s) so half the excess height (if any) is below the line.  For instance, if the o is 2mm taller than the i, put 1mm of the bottom curve below the line.

4.  Leave any letters with a 'tail' (g, j, p, q, y) in place, but glue them down last.  When the others are in place and the glue has set so they won't move easily, lay a ruler across the top of the 'footed' letters (m, n, r, t, v, w, x, z), but ignore the tops of the rounded parts (m, n, r) and just go by the parts with the flat tops.  Now glue them down so the flat tops are in line with the other flat tops.

I thought there would be some visual guideline for doing this, but I can't find one online.

If all this seems too complicated, just arrange them in a nice 'bouncing' design and glue them down.

Have fun!

Sue

A couple of coats of emulsion/household latex paint will give you a great foundation for your acrylic paints. Use up any leftover tins you may find lying around your house.

Latex and acrylic are variations of the same materials, and are compatible.

Sue