Is it possible to instruct a legatee as to what to do with the money? My daughter is a spendthrift and I would like the major part of her inheritance, (if there's any left!) to be placed in a pension fund.
If you include the "wish" it is not binding. You could place the money on protective or discretionary trusts (as the poster above says) so that she does not spend it all at once. Otherwise, an instruction is practically worthless.
When necessary I update my simple will, (which is based on one paid for by my employer who later discontinued the benefit).
Is creating a trust something I can do myself.
I spent a lifetime in insurance so have a fair idea of how to go about a revision.
Why don't you talk to a solicitor? By the time you die your daughter may have changed her habits, or she may need a lump sum for something genuine, rather than putting it in a pension fund. which might not necessarily be in her best interests. Setting up a trust fund and giving your trustees some specific written instructions of your wishes seems a more sensible route to me.